Grats on your purchase 
Also on the 450D here, with a 6D on the way. Looking forward to it.
I would agree to above, that the AF hunting is probably the nifty fifty.
I'm also upgrading form 450D to 6D in a couple of days. I find it quiet funny that a lot of people are switching from 450D -> 6D just like me/you.
I've tested pre-production and production bodies with 24L, 50L, 85 f/1.8, 28 f/1.8 and 70-200 f/4 and they were pretty damn responsive (on the center AF point). When you use lens with USM then focus is very quick and accurate. I find off-center AF points as good under good light. But I think that photography is also about knowing how to shoot, knowing the limits of gear and how to overcome them ;-)
Ofc 5D Mk III has more advanced AF and it's more "all-in-one" camera but 6D has enough features for me. And my next investment will be L glass (24-70 L II probably in a year).
Also 50 f/1.8 is known for focus hunting and has problems with focusing on objects that are further from camera.
I'm really looking forward for exploring FF and true focal length and DoF of my primes - especially 85 f/1.8 USM which is a bit tight for indoor shooting on APS-C. Should be good combo for portraits :-)