IRC - Internet Relay Chat - is an old technology, and doesn't get a lot of interest in a world with Skype and Google+ Hangouts, and Virtual Tabletops, but it has its own compensatory features that make it still a very viable medium for roleplaying.
I have been running games over IRC since 2003. That year I started a StarCluster game that finally wound up last year, after its 11th season. In 2006, I began alternating it with In Harm's Way: Napoleonic Naval, each one running for six months before changing. That game has been running now for eight years. I have run many other, shorter arced games as well over the years - from one-shots to six months at a time. In other words, I have a lot of experience in the medium.
Things IRC does *really* well:
1: Since IRC is a text medium, your graphics are limited only by your imagination. This is very much like a game played over the tabletop - no computer-generated scene can ever be as well done as your imagination!
2: No one can see or hear the players. All you see is the text they type. There is no cognitive dissonance between the sweet young girl character and the bearded, sarcastic player. The player is not part of the mental image, just the character. This leads to easy, deep immersion in the characters and their world. This is actually better than the tabletop.
3: GM-Player communication is generally over side channels directly between the two - this talk does not interrupt the flow of text in the main channel, and keeps that environment very immersive. This goes as well for player-to-player communication.
4: You can open another channel and roleplay two scenes at once, each with part of the group. The GM can switch between them as needed.
Things IIRC is not so good at:
1: Speed. The pace of an IRC session is slow. Typing is far slower than speech in the first place, and if you have slow typers, respons speed can be very slow. Figure on at most half the speed of a face-to-face session.
2: Transposed replies are another typing-speed based problem. The reply to a sentence can come after a second or even third statement is posted. This is usually the result of having some faster and some slower typists in the mix, but it can also be the result of GM and players being active in a side channel, and not getting back to the main channel fast enough.
3: Positioning can be iffy when you are not used to abstract positioning. There is no common view of the field of play, as there is with a battlemat or whiteboard, because of the inherent limitations of text. Use more abstract positioning, though, and you should be fine.