I've posted a few events, and I like the simplicity of the system. It doesn't try to lock in people and sell them a bunch of crap like Evite. You can setup private events as well, although I'm not sure how that's handled, since I assume anyone can read your RSS feed.
I do have concerns about scaling of the system and about privacy, but I'm enthusiastic enough about it that I'm going to continue to post information about film screenings and other cultural events that I find interesting. However, it looks like the developer is actively refining things and adding features, and this could turn out to be really, really cool before long.