When I got my XT,I got a body only.I had researched the net for a good walk-around lens for the purpose of learning to use the camera.With some reservations about focus issues,I bought a new 18-125mm Sigma.It was fairly sharp,but suffered from focus problems.It was noisy(no big deal),but worse,focused erratically.
So I went to Wolfe and compared it directly with the 28-135 and 28-105 f3.5/4.5 Canon lenses.These two lenses were both excellent focusers.And in the store,I took many photos with all 3 lenses.Both Canons were sharper than the Sigma.Other than the little extra reach and the IS,I thought the 28-105 was a better lens.So I sold the Sigma and bought a used 28-105.I wish I had known about this little gem from the start.It focused dead-on and instantly.It was sharper than the Sigma at all comparable focal lengths.It is compact,light enough,well built,has the "real"USM motor anf full-time manual focus....and definitely is a price bargain.It's my main lens.
I recently got an "L"(70-200f4) and now know what a really excellent lens can do.That makes me want the new 24-105 L.But at $1250???It will take a lot of convincing as capable as the 28-105 is.
Bottom line is that this lens is wayyyy underappreciated.I guess that's because it has been around a good while.This lens is.IMO,the best price/performance consumer lens that Canon makes.Very highly recommended.