I have found the same thing. I have taken (for my own testing) before and after shots with AO Enabled and Disabled, it is certainly not a perfect implementation but when it works, it absolutely adds value to the scene and is an excellent addition by Nvidia in their drivers.
AO is perhaps the forgotten IQ setting that people seem to just ignore (perhaps due to performance hit) or don't know about. Sometimes It can lead to a funky implementation like in WoW but for the most part it works and, in my opinion is just as important as AA and AF in terms of impact on visual quality. If only it worked in nearly all games like AA/AF.