Most lenses are at their sharpest around F8, so you're never going to get brilliant sharpness at f1.8. Also remember that DOF is very shallow with that wide an aperture, so you have to make sure your focus is flawless and that neither your camera nor your subject move between focus and shot.
+1
The lens has to be stopped down a bit to get good sharpness, say at f/2.8 or so. Sharpness will improve when stopped down further.
As for the 28-135:
The 28-135 is a very fine all-around lens. Stopped down, it is quite sharp. The IS comes in really handy in a lot of situations. I mostly used this lens if I want to take just one lens with me, i.e. on short trips.
On both the wide and the tele end, it is a little soft though. On these ends, it has to be stopped down for a sharper image. I don't find sharpness a big problem with this lens, IMHO it could do with a bit better contrast. OTOH, the contrast can mostly be corrected easily in PS.
For low light/available light work, it may be a bit too slow. If I remember correctly f/5.6 sets in around 85mm or so.