I bought a used copy to replace my Tamron 28-75mm, which I found myself increasingly unhappy with. The Canon 28-135mm offers extra reach on the zoom end, which I use a lot. It was my first IS lens and has converted me; IS is a huge help, especially when shooting in low light, which I do a lot. The range of useful shutter speeds is extended by quite a bit.
My copy suffers from zoom creep, which is annoying but doesn't really impact performance. It is very sharp and required no micro-focus adjustment at all on my 50D.
My only real complaint is the variable f-stops across the zoom range; if it were a constant f/4, it would be a perfect lens. However, f/5.6 at the long end is sometimes just too much, even with the IS.
For anyone who wants a dependable walk-around lens with good range, this is the one to buy. While I'll someday upgrade to the Canon 24-105 IS, I doubt I'll sell this lens, but will use it exclusively on my second body.