Mine fell apart when I pointed it down some steps to take a picture of someone below me. The whole front just popped out. I tried to push it back but it kept coming out again. It was past the warranty date so I took it into a shop to find out how much it would cost to fix it. They sent it away and a week later told me it would be £115. So I just took it home, stuck it in a cupboard and left it there.
A few months later I got it out to check out if it could be fixed. I pushed the front in and twisted it and it kind of jammed there, but it still autofocused. I carried on using it and got quite a few good shots out of it. Some months ago I found a 2nd-hand one on sale for quite a bit less than full price (can't remember exactly how much) and bought that. It didn't give me any better shots than the broken one but I carried on using that one. I would expect it to fall apart at some point in the future, though.
In my opinion, it's just a shoddily built lens that happens to create fantastic pictures. Canon should bring out a Mk III with a proper casing and metal bayonet fitting and charge about £150 for it. I'm sure they'd sell a lot of them.