I've been eyeing the 40 2.8 for awhile now and finally gave in tonight and bought one. I saw that Best Buy had them for $150 and figured I may as well give it a try. I can always return it.
Anyway, shot about 100 shots with it tonight and I'm very impressed. The only other prime I've ever owned/used is the nifty fifty and I wasn't a fan. It was loud, slow AF, and never really cared for the IQ. The 40 tho is dead quiet, fairly quick, small and light, and tack sharp. I'm using it with my 7d. I'm also really liking the idea of shooting with a prime. I pretty much just point and shoot and don't have to think about zooming, etc. . It's really making me want an 85 1.8 now
If you haven't tried one yet, I recommend it!!