Starting today there's a brand new feature on Google+ which works inside the photo app that'll make it much easier for you to tag the friends in your photos that Google's recognition engine believes match those you've already tagged. This feature is completely opt-in and wont tag anyone unless you flip the on-switch, and as is natural this day and age, Google has made it abundantly clear that they don't mean to try to swallow your identity up with a barrage of I KNOW YOU alerts. You'll upload your photos as normal, see that Google+ has some suggestions for you, and say yes or no to each of them.
This will remind Facebook users of a very similar feature automatically added in to their photo uploading sequence over the past few months. Your humble narrator's experience with said feature has shown quite clearly that Facebook thinks that my baby girl is actually my wife's sister - almost without fail. So that's weird. Google's recognition engine might have a bit less luck than that, believe it or not, even if their software finds a way to be better, simply because the photos I and the people I converse with on Google+ neither want their own image on the network or have any images to be recognized up there in the first place - so we'll see how it goes.
Meanwhile we've got other facial recognition functions out there from Google right this second that have had less than stellar records. The one that has popped up most recently and most prevalently has been the facial unlock screen included on the Galaxy Nexus - and on future Ice Cream Sandwich phones if it does well enough in the interim. Of course that's another opt-in feature, I'm sure you're well aware.