Pros:
1. I have only used it for photos in sunny landscapes with my Canon 450D and it improves images dramatically!
2. The glass is thin so no problem with a bulky extension at the end of your lens
3. It comes in a transparent plastic box that closes pretty much tightly so you can keep it safe there
4. It withstood a drop from about 1.30m
Cons:
1. More of a lens problem (the 18-55 that comes with the 450D) because when focusing it rotates the part where the polariser is attached so you may have to come back to re-adjust it; plus I get this feeling that even though it is screwed with several turns I have probably unscrewed it back and it is about to fall
2. I bought it in a cost per photo basis, so I went for the best one I could afford but when I dropped it once I thought I had spent too much in something that I can break very quickly.