Somebody on boards.ie says that it's basically (or actually?) the same as so people might want to be aware of of using such a system before plunging in.
I don't think there's a directory though. You need to know who you're calling before you call them - if you know what I mean. You maintain your own contacts so there's no notion of a distributed database to be maintained. I monitored connections in and out and didn't detect anything untoward.