i like using yahoo except not that many people use it.
pros:
-you can be "invisible" where your still online, but it looks like your not signed on
-there are different busy/away signs (not at my desk, stepped out, etc)
-you can still chat while you have your away messages up. unlike aim where you have to take your away message up and people bug you
-you get alerts when people add you to thier buddy list
-you use webcams on here (not sure if you can use webcams on aim, never tried it)
-you have IMVironments where it can be interactive (you can doodle and play tic tac toe with your buddy
)
-you get IM's when your logged off
-you can save chat convos
-some others but i forget right now
cons:
-not many people on it
-if you want to add someone to your buddylist but don't want them to know, they will know cause it sends them an alert saying you added them (the part i like the least
)