For landscapes, portraits , and general center point usage, I find it to be very good.
In terms of AF accuracy of the center point, its extremely accurate.
In low light, it matches or exceeds my 1D MK III and is better than the 1D MK IV
Outer points and subject tracking are its weak points.
The lens you use is a big part of AF speed, but tracking, low light, point selected, and accuracy vary by model.
I agree the center point is accurate, I do think using an f2.8 or faster lens is essential on the 5d2
but i do find my 1d3 AF to be considerably better in every respect than the 5D2 especially using f4 lenses such as the 24-105 f4L or the 300 f4L