Ask HN: Mass SMS text message services?

I'm asking a question on behalf of my brother who runs a youth non-profit org in Providence, RI. He's looking for a way to send text messages, en masse, to an opt-in network of constituents (mostly teens).

Can any recommend any good services to handle that?

He's looked at the follow:

http://www.sumotext.com/

http://www.eztexting.com/

http://www.txtmobster.com/

He's also looked into working with carriers directly. I really don't know what is the best way to handle an opt-in, group text alert list, or if any of these companies are reputable, so I thought I'd ask the HN community.

Thanks!