Pros: ¤ 240mm Radiator (See Cons)

¤ SILENT Pump

¤ Great Cooling (See Other Thoughts for Details)

¤ Plenty of Screws for any mounting scenario.

¤ Amazing Packaging detail. (See Other thoughts #2)

¤ Idiot Proof Mounting Brackets for AMD, and Intel. (See Cons.)

¤ Simplicity in Design (See Other Thoughts #3)

¤ Blue LED

¤ Rubber Fan Silencer (See cons)

Cons: ¤ Radiator MOUNT is aluminum (Or at least it strips as easily...Yeah...that was pretty stupid in my opinion, Cooler Master. I had to modify my case slightly to get the rad to fit, and apparently didn't sand smooth enough so when I tried to mount the fourth screw on my 240m is stripped the contact. LUCKILY I'm an insane DIY'er and know many ways to fix this, but the average Joe may not; and the MOUNT of a LIQUID RADIATOR should be secure.

¤ The AMD Mounting System...This is both a pro and a con, so I've listed the cons here, and pros above:
Cons: Nearly IMPOSSIBLE to mount by yourself without doing some SEVERE Jerry Rigging if the motherboard is in your case, and you have a cutout allowing you to put it on with cable management room behind it. Unless the motherboard is lying flat on something the install will be nearly impossible since there are pressure fits on each side of the screw/mounting bracket.

¤ Radiator: The radiator could have been thicker. Although it does do a GREAT Job at keeping my 1100t cool even at 151 watts of draw I just love the ability to do better things with a thicker rad. The Fold spacing on it is great, but does amount to a LOT of coil Whine when in full speed.
It also would have been nice to have known as this was my first time jumping into Water Cooling that a 240mm Radiator is NOT the same setup as two 120mm fans that are side by side. On the radiator the fans sit nestled side by side without a gap, whereas in most computer system (MOST) they have a small gap between them. This means that there's only ONE way to make the radiator fit your case, and that's by tearing into the steel that the case fans were mounted to in order to make the holes larger.

**NOTE**DO NOT BE "That Guy" AND BORE HOLES/SAND/CUT/GRIND/MODIFY YOUR CASE AT ALL WITH COMPONENTS INSIDE! Steel is HIGHLY conductive (EVEN IF IT'S PAINTED ONCE YOU SHAVE A LAYER OFF THERE ARE PARTS THAT AREN'T PAINTED AND WHO'S TO SAY THAT PAINT IS A GOOD INSULATOR?), and if so much as a tiny flake lands on your stuff you'll be sweating bullets trying to put out the fire you just created by trying to shave 30 minutes off your project time. Not to mention a very sad bunny once you figure out that the wonderful radiator you just bought caused you to have to modify your case, which in turn made you lose your precious 600 dollar graphics card that isn't covered by accidental shortage. Don't be lazy, do it the safe way, sand when you're done boring, and make sure to dust the case out, and use a neodymium magnet, or the likes to get all the small parts that may have magnetized from the grinding, and are loosely attached to your case via electromagnetic field.

¤ Coil Whine: When this thing hits full speed believe me you WILL know. The computer it's in is in a fairly crowded office with a lot of furniture, two desks, and three computers, and when it's on full blast I can close the room door that's approximately ten feet away from it, walk about 5 ft aw

Overall Review: 1. The Seidon 240m Keeps my processor amazingly cool. I am running an 1100T and was testing a 4.5Ghz overclock while running a mere 42c @1.5 Volts when my motherboards VRM's decided to pop because they'd gotten so hot. In other words, this kept my processor cool enough to COMPLETELY make me forget about my VRM's which were getting so insanely hot trying to maintain that they literally burst under the pressure. Granted, they're cheapo VRM's, but that board (MSI 870s-G46) had taken an Athlon II x4 640 to 4.0Ghz without breaking a sweat. I guess the phenom was just too much for it though.
Ambient temps in my case average around 29-35c seeing as the room my system is isn't very well insulated, and is also beside the garage that is a heat trap in North West Texas.
Temp idle is roughly 29-31c while under load it tops out at an amazing 35-40 depending on ambient room temp. This is while running standard 3.7ghz Turbo Core since the spare mobo I have can't handle a small overclock at all.

2. Even my fans were in bags, although I'd have liked to have seen them wrapped in foam instead. However, the design of the fan fins pretty much prevents any foreseeable damage occurring while in it's package thanks to the way they're set in the box.

3. The Design of this all around is nice. I like and appreciate the industrial-type look that Cooler Master gave this cooler, and even more so LOVE the fact that they give you a fill port for later use if you're past your warranty. What I DON'T love about the design though, is the incredibly rigid tubing. Who in their right mind thought that PLASTICS were a good design for coolant to traverse through at any amount of speed? Granted Rubber hoses break down, but they give you flexibility that plastics just...can't. The rigidity of these hoses is absolutely ridiculous. I can barely get a 30 degree turn on them to the top of my case ( Sentey Abaddom II) Granted, some of that is my fault as I've got a case that's NOT radiator friendly in the top space, but there's still a good 6 inches of room and rubber would have worked a hundred times better in my scenario.

4. Coil Whine: While it IS pretty loud. (Sound meters put it right near 76db from 1ft away) it's MUCH more tolerable than the 60db levels of static wind pressure coming through grated fan grills for sure. I'm QUITE used to the friendly hum of a well cooled computer, and am not impartial to it by any means; but the coil whine on the Seidon DEFINITELY makes things a little easier to handle without getting a migraine (FOR ME.)

5. Coil Whine: Really "intolerable" after about 65-75% speed and higher on the fans. The odds of you cooking your processor enough to NEED that are pretty low. Honestly unless you're running a VERY hot high end intel chip then it shouldn't need to get above 50% to effectively cool your processor. If the fan speeds start getting HIGHLY noticeable then odds are something is wrong, and it's time to RMA.

**EDIT 1/27/2019**
I performed a "tube-mod" on this about 6 months after purchasing. The tubing was 3/8" Inner Diameter, and 1/2" outer (iirc.) Oc(dot)net has plenty of information on how to do this.
Even WITH nearly 6 feet of tubing, and a swiftech MCRes v2 attached to the line this cooler did a great job across 3 CPU's, and 3 GPU's before I finally traded it to an older gentleman that wanted to give me an SSD for it so he could put it on a display shelf because he thought it looked "cool."
This was in 2016, roughly 4 years after purchase, and this thing was STILL kicking out water with zero issues.
It DID weaken a bit by then, but that's to be expected after all the crud I'd put it through.
To anyone on the fence about this cooler, you'd have to be nuts not to consider it.

Pros: -Completely self-contained, no maintenance
-Good heat dissipation
-Quiet

Cons: -Can be tricky when trying to install the unit into the case/onto the MB, but whaddya gonna do?

Overall Review: First time using a liquid cooled system. Biggest fear was that at some point it would start leaking or burst a hose. Maybe get clogged up and have no way of knowing it had lost it's thermal dissipation, burning up my CPU. But, I'm happy to say that after two years I haven't had one problem. This system is SOLID!

Pros: It is quiet when using a windows application.

Cons: Very Noisy when playing a game! The pump makes a rattling noise, like a HDD, but much louder.

Overall Review: Has been doing that since I installed it more than a year ago, it is getting worse. I have to open the computer and shake the hoses for the noise to go away.

Pros: - No hassle, easy to install
- Super silent
- Impressive cooling

Cons: - None that come to mind- aside from the fact that it's out of stock

Overall Review: - Would be great if they had other radiator/fan sizes. Would like to see a 2x140mm version

Pros: Kept the CPU cool, while it worked.
The fans still work.

Cons: Stopped working almost 1 year after purchase.

Overall Review: Had to buy a Corsair cooler at Best Buy to replace it because I needed my computer then.

Pros: -Quiet
-Cools well
-Looks great

Cons: -Cost more when I bought it, but has come down.
-Fans aren't "pretty"

Overall Review: I used this both on my 4670K overclocked then modded my R9 290 Reference card to use it and it cooled it overclocked to 1200Mhz to 50c max load while operating quietly.

Pros: It works as advertised; keeps CPU temps at reasonable levels (NOTE: I am not OC'ing).
Straight-forward installation (HINT: spend the time to watch a couple of YouTube videos showing how to install; it will help tremendously with installation because the instructions are not the greatest)

Cons: None except for the instructions being a bit lacking.

Overall Review: Config:
MSI GeForce GTX 970 Gaming 4 G
EVGA 220-P2 1000W 80 Plus platinum power supply
Cooler Master Siedon 240M CPU cooler
G.Skill Ripjaws 4 series 64GB (8x8GB) DDR4 2133 SDRAM
Intel Core i7-5930K
3x Seagate Barracuda STBD3000100 3TB 7200 rpm HDDs
Cooler Master Storm Stryker case

Pros: None so far

Cons: My equipment was missing the motherboard standoffs. For some reason, Cooler Master tech support is unable to tell me the height of the standoffs so I can get replacements. They are also not able to ship the parts in a timely fashion, though their rep keeps assuring me they'll be sent "tomorrow."

Overall Review: Never bought a cooler master product before and thanks to my experiences I shan't be in the future.