Pros: This thing has been nothing but drama - connectivity issues! It's stumbled for the last 6-9 months and has finally failed. FAIL

Cons: Cannot withstand moderate use

Pros: I purchased two of these enclosures, one back in April of 2013, and a second one a little over a month ago.
The enclosures are pretty quiet and the drives are kept fairly cool due to the large fan in the back. Not having a external power brick is definitely a plus.
The enclosures are well built, and the drives are easily replaceable thanks to the drive brackets. The drive slot LEDs were not blinding like some of the others out there
The clean look and the reliability were what made me purchase a second one, even though there were other less expensive options available from some of their competitors.

Cons: Not really a con, but it is a little pricey for the feature set. However, the enclosure seems better built than some of the others I've worked with (like Mediasonic). The quality is definitely there.

Other Thoughts: Both of my enclosures are connected via eSATA to a HighPoint RocketRaid 642L controller in my Ubuntu-based virtualization server. The first one is populated with 4x 1TB Seagate drives, and the second has 4x 2TB Hitachi drives.
One RAID5 volume per enclosure, with multiple virtual machines using 'slices' off each volume for the OS disk.
Performance is about what you'd expect from a eSATA connection.
Some of the other reviewers mentioned the drives are not hot-swappable in this enclosure, and that is not entirely true. It depends how you have the drives attached to your system: if you're using USB, that's expected. If you're using eSATA with a good disk or RAID controller (and not just a cheap port multiplier), then the drives are completely hot-swappable. I've tested this several times, and every time I was able to remove disks from the RAID group without causing a complete rescan of the bus.
Your mileage will always vary depending on how good the hardware you have may be.

Pros: The Icy Dock MB561US is a well built, solid enclosure.
Fairly quiet operation. (not silent but certainly acceptable)
Transfer times are very good over USB. Did not try eSATA
Very easy to install drives and configure.

Cons: Unit failed after two weeks of very light use. I installed 4 x 1TB Seagate drives and had it attached via USB to an Apple Time Capsule for additional network storage. In this configuration, it appeared to spin the drives down when they were not being accessed but there was still an audible "whine" from the unit. After only two weeks of service, it failed to mount more than 2 drives at a time.

Other Thoughts: I purchased this unit from Newegg and contacted them for an RA. I received the RA, boxed the unit up and shipped it to them (on my dime). Once they received it, they contacted me and said they would not honor the warranty replacement because the UPC label had been removed from the box. I did this per the manufacturer's instructions to fulfill a rebate promotion (which I also never received).

Pros: Well constructed body. The enclosure keeps the drives cool and runs quietly. The metal latch has a nice look and works well. A good price for a solid enclosure.

Cons: One connector of the included eSATA cable arrived broken or broke when first plugged in to the port. This caused the OS to recognize the drives, drop the speed to 1.5 Gb/s and then error with a 'BadCRC Handshk' error. The drives were recognized with the USB connector. I purchased a new eSATA cable and the OS recognized the drives using the new cable.

Pros: I bought this as an expansion for an HP MediaSmart EX495 home server off it's eSATA port. The HP eSATA port is already a port multiplier so this worked out great. No problems doing setup and loaded up with four Segate 1.5TB drives just fine. Fan is quiet on auto.

Cons: No firmware updates.

Pros: - Slick quality design
- USB 2.0 and eSATA
- Adjustable fan speeds
- Easy to install

Cons: None.

Other Thoughts: I purchased this for my home theater attached to a WDTV Live player running wdlxtv (b-rad) firmware that supports multiple hub drives. Four 2 TB harddrives in each bay packed with blu-ray rips (.mkv) with thumbnails of each movie in their own folders. Hooked it up and let the WDTV read the drives and everything worked perfectly! 8 TB of high-definition movies all in one unit. Transfer movies to the Icy Dock is easy through WDTVs FTP which sees all the drives. The hot-swappable issue is not a problem since I'll be keeping my harddrives in the Icy Dock all the time. I love this product! If you have a home theater and want to keep all your movies in one unit - buy this! You won't regret it!

Pros: Can connect to my router and use it as a mini server to access all my movies, pics, music from any DLNA device in the house. Very pleasing to the eye, size is right for where I needed to put it. Built in power supply no power brick, fan runs nice and quiet.

Cons: all drives cycle when hot swapping a drive. Comes with trays that are solid in the front and does not allow great air movement. No raid functions.

Pros: I thought this was going to be a great purchase for being able to remove hd's individually while the IcyDock was still powered on (Hot Swap). I planned to make this a backup unit so I could store one of the disks off-site. Take it out and plug in as I needed.

Cons: But when I tried to do this, all 4 of the hds "recycled" or unmounted and remounted. If one of the other hds had been writing data, it would have been lost. This was added to their website after I bought it.
Note: Due to the chipset designs, the USB host will refresh while swapping a single hard drive in JBOD mode via USB. The remaining drive(s) will cycle during this process, please be sure that all active tasks are completed prior to swapping.

Other Thoughts: I wish I knew if the same thing happens with eSATA connections.