I have one too.
Consider a UV filter not because you want to protect against scratches. I believe UV filters degrade image quality (the extent of which depends on what kind of UV filter).
The best in calss ones you should look at are
- B+W F-Pro MRC
- Hoya HD
- Heliopans top of the line (whatever they're called).
I use the first one on my 17-55, but the second goes on my 11-16. Both are awesome filters with almost no noticeable degradation in IQ. Also they cost an arm and a leg at $80 each.
The reason you want to buy a UV filter especially for this lens is because there is a known dust issue where dust enters the front element. Some think its through the zoom mechanism, but I think the zoom mechanism itself is the one that triggers a vacuum/dust pump effect. The front isn't terribly well sealed and why else would typical dust reports show dust behind the front element?
Now you can clean this as there are tutorials online since this is a popular lens, but I for one hate doing that and I'm a clumsy guy. It's not too bad because I've done it myself, but ever since slapping a filter on mine, it's been dust FREE.
My complaints about this lens:
- Older IS system that fails more often. The 15-85 employs the new 4th gen IS
- build quality is so-so
- Zoom creep. Seriously? Zoom creep. That means if my camera is pointed down and if its not locked in at 17mm, and at like 24mm, it the glass will pull the lens out to 55mm. Meh. I guess its a heavy lens because there's a lot of glass, but you know most L lenses don't have this.... maybe the 24-70 though.
BTW I love this lens on my 7D.... or even when it was on my Rebel. It's my main walkaround lens and stays mounted 75% of the time. Except when I need more reach or if I'm going UWA. I don't even touch my primes anymore with this + flash. Gorgeous lens. To be honest it's not a good value though, but since I bought it used I figure I can sell it with minimal loss. I wish you got a little more for this $1000 lens, or it could be a bit cheaper. The IQ of the 18-55mm kit lens is already excellent.