IM doesn't need to be pigeonholed in either category, it all depends on how you use it. The Telephone itself can be a senseless waster of time in the wrong hands, but that doesn't take away from it's usefulness in the right hands. IM is the same way. I use it frequently, and it helps to keep long-distance friendships alive which might otherwise die, as well as giving me quick contact with friends and family when needed. And, as mentioned above, it's a lot cheaper than a long-distance phone call.
Any communication medium can be a senseless waste of time in the wong hands, including message boards *cough*Lumberjim*cough*,
but that shouldn't reflect badly on message boards in general. IM has it's benefits and drawbacks, so it's not for everybody, but when it does work, it can work very well.