Our group has been playing over IRC for a while, and there are some definite benefits and drawbacks. Being able to scroll up or check previous logs helps keep everyone on the same page of what the plot is, and I find the DM can weave a more effective metaplot, since even if we didn't pay attention to the subtle hints from 4 sessions ago, it's possible to look back for them once you have information. For high Int/Wis characters, it's a great boon, since you don't have to keep meticulous records, or sheepishly ask the DM "Um... what was that again?" when you're trying to remember something your character should know.
I also find that online games allow for more subtle roleplaying (at least for fast typists). Unlike live games, where only one person can really speak at a time, and everything is made up on the spot, you can take a little more time to think about how to say something in character when it's being typed.