"AO gives soft area shadows similar to the effect you see with GI or HDRI." Oh my god! Why the hell did they take the trouble of making GI and HDRI then, (knowing that even with Brazil / Vray/ Final Render/ Maxwell), it takes hours and hours to get a good GI render? AO lets you simulate GI and HDRI to some extent, Lets just say AO is the Poor Man's GI /HDRI :)