I decided to give this lens a go, after hearing and seeings its IQ I couldn't resist. I have a sigma 150-500mm and, on my t2i, it gave me good results stopped down to f8 but on my 7d it seems lacking. I know the sensors are VERY similar if not identical, but it just doesn't seem to resolve well. I use this lens at 500mm 99% of the time so i know for a fact switching to the prime will not inconvenience me. If I really need a shorter focal length while using the 400mm, I'll have my t2i with 80-200mm f2.8 L over my other shoulder. Since I upgraded to the 7d and all of its features, I owed it to myself to get the best glass. the focus on the siggy is sloth-like compared to the prime, sometimes I want to shoot myself. (I tried my cousins copy a few months ago).
Hopefully it gets here by the weekend so I can put it through its paces! I'm stoked, just in time for the birds to migrate through here. I also found a pair of eagles yesterday, no close photos yet though.
Anyway that was a little rant, i'm excited!!! 



