Pros: I am new to DSLR photography and about 1 year ago purchased the Canon T1i Kit. I then purchased the Canon EF-S 55-250mm f/4-5.6 IS Telephoto Zoom Lens for outdoor shots. The pictures were fine and blew away any of the point-and-shoot pictures I have taken over the past 15 years. After reading many reviews on the Canon "L" Series lenses I decided I was going to upgrade to all "L" Series lenses. The first "L" Series lens I purchased was the Canon EF 70-200mm f/4L IS USM Telephoto Zoom Lens mainly because NewEgg had a great price on it. I am so glad I made the purchase. The outdoor pictures are stunning and I can't wait for the weather to break to take pictures in the spring and summer. The next lens I plan to purchase will be the EF 24-105mm f/4L IS USM Standard Zoom followed by the EF 17-40mm f/4L USM
Ultra-Wide Zoom or the EF 16-35mm f/2.8L II USM
Ultra-Wide Zoom to complete the trio.

Cons: None so far. I do have to purchase another camera bag for all three lenses.

Other Thoughts: I need to purchase some additional lens filters and eventually I plan to upgrade to a full frame camera which is another reason in purchasing the "L" Series Lenses from Canon.

Pros: There are so many in depth reviews of this lens out there I though I would keep it simple. This lens shines in every aspect from image quality to speed of operation.

Cons: Its not as fast (aperture) as its big brother (f2.8l mk I and II).
More expensive then third party alternatives.

Other Thoughts: This lens is one of the best lenses available for slr's today, because it is excellent in image quality, ease of operation, speed of AF, build quality, etc.

Pros: VERY VERY SHARP!!!! I can not believe the detail I see when blowing up the images on the computer. I had no idea my Rebel Xsi was capable of taking such amazing pictures! I took a shot of a chipmunk about 15 feet away, wide open at 200mm handheld (Image Stabilization ON). I was able to zoom in on the computer and see every hair, whisker, toe nail, etc. - I almost couldn't believe my eyes!

Cons: I've had my eye on the 5D Mark II, but it's going to be difficult to justify a camera upgrade with the images I'm getting from my Xsi and this lens:)

Other Thoughts: Few products have impressed me enough to take the time to write a positive review. This lens is one of them!

Pros: long range, fast focus, image stabilization, weight, built

Cons: price, small aperture

Other Thoughts: This is my 3rd Canon lens and I like this a lot. Even though it's not a fast lens its zoom range and image stabilization make up for it. It's hard to capture moving objects at night but during daytime you can do a lot with this lens. You can even take pictures of strangers unnoticed.

Pros: This lens has almost everything you could ask for in a mid-range zoom, with the "L" glass producing truly pristine images. I decided to go with this one instead of the 2.8 and although the extra f-stop would be nice at times, paying $xxx less as well as reducing the weight by almost half by carrying this lens instead has been worth the trade-off. Additionally, the IS lets me get away with hand-held shooting in many situations that would potentially demand a tripod if using a lesser quality product. In all, if you can justify the purchase of this high-end lens, you will not be disappointed.

Cons: Would be nice if the tripod ring mount were included in the box, but that is not a fault of the lens itself.

Pros: 99% of my photos are taken outdoors in fairly good lighting conditions. I thought I would try the f4L to save a couple bucks over the 2.8. I have found that for my needs the f4 presents no real problems for me. I am usualy able to use a tripod, but the IS dose come in handy for that quick unplaned shot, and dose work well when you need it. I'll be keeping this lens, no need for the 2.8 for me. Auto focus works great.

Cons: how could you have cons, its L glass.