I currently use both MSN and AIM. I downloaded Yahoo several months back, and it didn't seem to work to well for me. Who knows, maybe it will be better for you! I guess I just prefer MSN because it doesn't seem too complex!
I use MSN, Yahoo and AIM...but I am more often than not unavailable all around so I guess chatting doesn't happen that often unless you don't mind a quickie with me.
I have used ICQ, MSN and now using nearly only Yahoo... for webcam its the best when you have a firewall or a router and you can send message to offline poeple (like ICQ) but not MSN.