I've actually tried getting used to using Av on the 5D, which I will say, using Av on the T1i was a lot easier. Mainly because I'm so used to using manual now with independent aperture controls, that I find it counter intuitive to change my aperture with the upper scroll wheel. However, if in Av on the 5D they had it so that the thumb wheel controlled aperture, and the scroll wheel controlled exposure compensation, it would seem more proper.
I'm probably the only one that thinks that, but who knows?
So because of this, I shoot in manual 100% of the time.