Well, for our purposes a group based chat is much, much nicer.  Especially with logs.  People drop to IM at times (or private messages in IRC), but we seem to get more out of the virtual watercooler/mass meeting room.  The one on one stuff generally works better through email, where it's less transient and people can easily reference it later.<p>If you have people who haven't touched IM before, you should definitely checkout Campfire.  One mass "company" room can get crazy so you can always fragment it off into a couple of area specific chats alongside the primary.