"low-bandwidth web app"?
They deliver "up to 140 character" messages, but delivery on their website requires loading quite a bit more, and doesn't even display anything when javascript isn't enabled. Even then it often hangs and doesn't deliver.
Which is to say, I don't think I'd trust them reinvent sms in a sensible way on any phone. The idea isn't that bad in the loopy twodotoh way that substitutes for logic and/or common sense these days --they reinvented sms for the web, and now are bringing it back to the original sms platform-- but they'll be burning lots more bandwidth in the process, running up costs for the carriers, and possibly costing them sms revenue. Not sure if, on balance, the latter will be amused.