I really liked the range it offers. I used to use it on a film body (which made it a solid every-day lens), and on my 10D when I first got a digital. On a crop body though, I found that I needed to carry a wide angle as well. Eventually I stopped carrying the 28-135 as I shoot a lot of wide shots. Another great feature is that it takes semi-macro shots. The IS is ok (not as good as newer L series IS), but certainly convenient. Image quality overall is pretty good.
I also had a lot of problems with the lens. I work in pretty rugged places under harsh conditions, and the auto focus broke after about a year. Also, since the lens extends, it attracted a lot of dust which I can see inside the lens, but it doesn't seem to degrade image quality. The build quality feels a bit fragile and the lens doesn't do well in rain or dust so consider this if it is a factor for you. I felt that the lens was a bit expensive when I got it, but this was when it first came out so perhaps the price is more reasonable now.
Sorry, I don't have any experience with the Tamron, but good luck making your decision!