It allows multiple logins on multiple services, which is useful. It has the most horrifying default dock icon known to man--an animated duck that flaps its wings when a message arrives--which makes the app virtually unusable until it is replaced. Fortunately, there are many icon sets available and one can replace the dock icon with a non-animated version.

Adium will do until a native gaim port turns up. It would be much improved if there were an easier way to turn off the dock animation.