A thread at WebmasterWorld named Gmail spam detection is utter garbage. Some complain that the spam filtering is no good, some say it works wonders. False positives and false negatives.
Think of it this way. More people that Google signs up, more real email that goes through the system, the better they will get at spam detection.
Personally, I would not switch my corporate email to filter through gmail. I use three levels of spam protection at this point. (1) Barracuda Networks at the top level. (2) Anything that gets passed that, and there are some go through Spam Assassin. (3) Now if that let's things through, that shouldn't go through, it might get caught by the built in Entourage Spam Filter.
And I check all the false negatives daily or so, its rare to find any, but some come up.