If you've ever tried to DJ a party, then you'll know how hard it is to match beats accurately to sound like a professional.
In the end, you might end up hoping that people are too drunk to notice but you can cheat with AccuBeatMix. AccuBeatMix basically assesses how much time each track needs to synchronize accurately with the next track. The result is that transitions are considerably smoother than trying to mix it yourself. That's not to say you'll notice a massive difference however unless you listen closely to the transitions.
There's very little to AccuBeatMix. Once installed, it becomes an integral part of iTunes allowing you to activate it by checking and un-checking a box in the top left hand corner. You've also got a volume slider to control the level of volume of the transitions. Of course, AccuBeatMix can't perform miracles. Try lining up a Rolling Stones track with one from The Prodigy and it's not going to work very well but if you can choose some suitable tracks, AccuBeatMix does the rest.
It would have been nice to have had at least some kind of preference panel or control interface for AccuBeatMix but it's certainly better than trying to mix yourself.