JMP is great! I ported over my number that I've had the longest to them probably 2 years ago when it was still beta. It's so convenient having sms on multiple devices and my computer. I have all calls from jmp forwarded to my phone with service tho. And I just switched that from AT&T prepaid to US mobile.
Edit: I also like popping in to read their group chat. Lots of nerd talk in there.
Also, I'm pretty sure if you ask their support you can still get a free XMPP server with JMP service via Snikket.