Interesting what Jeff says, I notice that too. I guess I tend to slow down and really think about what I'm doing with a prime, and take a step forward or back rather than just zooming. That said, I usually do use zooms (one reason I mentioned was because of dust and DSLR's). I usually use an f1.4 prime in low light but plan to use them more in some situations.

The slight difference in quality between good lenses is so slight that I don't worry about it. I don't think you could see the difference in sharpness unless you had a heavy tripod, cable release and so on. In real life shooting, it is just way too far down on the list for me to worry about. However, in addition to the barrel distortion that Charles brought up, I do see a difference in some lenses in how they render out of focus highlights (OK, I'm on the bokeh kick lately). The 50 f1.4 and 80-200 f2.8 (two-touch, non-AFS) are rather harsh - but the 85 f1.4 is beautiful (wide open) and the 28-105 is pretty good (and not a pro-level lens). This is one of the reasons I like the 85 so much, this quality is most important (IMO) at low light levels - which is when I go for a fast prime.