One INDISPUTABLE fact here is that MSE / Forefront client security definitions are updated ALMOST DAILY.
I don't suppose ANY of the FREE AV software out there can do this stuff as quickly as MS and even the paid for stuff.
So far I'm sticking with Forefront Client security -- (mse IS ESSENTIALLY THE SAME) as it seems to work just fine, has a tiny footprint and I've tested on a totally separate coputer trying to infect it and it detects just fine. I've also tested a local "rogue" website and it also detects.
Not exhaustive tests I agree but the fact that MS is updating the definitions almost daily can't be bad.