Back in the film days, the 28-135mm was thought of as being an ideal (mid-quality) walk-around lens. You are right that the focal length range is pretty good. Of course, there are wider range zooms...18-200mm and so on...but those really tend to compromise on quality, more than most photographers would like.
The 28-135mm does have a downside, and that is that it's maximum aperture is only F4-5.6 (or thereabouts). So you may have a hard time getting fast shutter speeds in low light. It's not a top quality lens, but it's not on the bottom of the list either.
Personally, I would prefer something wider for a walk-around lens. I've got the 18-85mm IS, which is pretty much the EF-S version of the EF 28-135mm. All the other comments apply to both lenses. It's a good, mid-level zoom lens but the max aperture leaves something to be desired.
There is a new one, the 15-85mm IS, which is said to be better...but I haven't heard too much about it.
I do also have the Tamron 17-50mm F2.8...it's one of my favorite lenses, and the one I use most often for shooting weddings etc. The large max aperture is great and it's optical quality is pretty good for the price. It's not as good as the Canon 17-55mm F2.8 IS, but it's less than half the price. There is a new version of this lens, with 'VC' (Tamron's version of IS), but it's a good deal more expensive.
So it really comes down to what you want out of a lens and what you would mostly shoot with it. If you are going to be shooting in situations where you'll need a faster shutter speed (hand held in low light, or with moving subjects) then you may be better off with the Tammy because of the max aperture. But if you're shooting in bright daylight, then extra range of the 28-135 (or 15/17-85mm) might be better.
