I'm astounded at how frequently people are apparently making breakthroughs in sending raw text between people. I keep thinking that's a mousetrap, and people somehow keep making money off of* better ones. Shows how much I know. * Or at least building huge userbases on...
Consider that many of their installs come from making pretty much any extant handset in the world (a large fraction thereof, including some very old phones running J2ME midlets) all interoperable across national borders and carrier networks. I don't know of any other way that this has ever been available across a such a range of devices and people. Add to that, zero sign up, account creation, entering server configurations, etc. There was considerable room for improvement on the status quo here, that's what they're built on.