I used to play all the time, but XBL has been taking up most of my online gaming time now. WarCraft III was my first real stab at the RTS genre, so I was skeptical to say the least about going online, but the match making in WarCraft III is awesome. My first time online I got matched up with someone in the same boat as me, first timer to the genre, needless to say 4 hours later it was on the best online experiences I've had.
Don't be afraid just to jump on and give it a try, I've found that most of the people playing are willing to give pointers on how to improve your game. It really is a heck of a lot of fun, :pulls out the WarCraft III CD: Mmmmmm... WarCraft III.
Warcraft III online is ok in regular mode, but go to custom games, where all the user made maps are. There are hero arenas, RPGs, Castle Madness, Commando Wars, and all kinds of awesome maps, with more being made and modified all the time. :Also pulls out War3 CD: Ooh, a Murder at the Mansion game!