I was going to do more research before I posted this, but since it appears that I've created a minor epidemic in my local community, I'm posting this a bit uninformed. I received an email inviting me to SMS.ac, which I would normally ignore, but it was from someone who's judgment I trust. I clicked through the signup process without finally completing it, but unwittingly gave the service access to my MSN IM information. This spammed my whole buddy list with invitations. It was unintuitive to unregister even though I hadn't completed the registration process. Also, I heard from someone that if you don't unregister, the service continues to send invites to people you add to your buddy list. Anyway, I have no idea if the service is interesting, but the the fact that you invite your whole buddylist before you actually try the service and the difficulty and deleting your account makes me skeptical about it.
Anyway, I'll post more information when I get it and apologies for sending invites to people to a service I'm not even using.
This is the first time I've used rel=nofollow in a post. ;-)