The device basically works as advertised: you can send short messages to friends, social networks, and a tracking web page. You can probably also get help in an emergency, although I couldn't test that. The biggest limitation of the device is that it is strictly one way, so there is no way of getting responses (combining it with a satellite pager might be a good idea).
It's far from perfect, though. The device needs to be paired, via Bluetooth, and that's a process that doesn't work well with every phone (likely due to software problems on the phones, though). And instead of a straightforward messaging application, you have to go through several steps on a web page and the application. In part, that's probably to accommodate the "dumb" versions of this device.
If you're traveling away from cell phone connectivity, but don't want to spend too much money, this may be a good option. It still isn't cheap: in addition to the device itself, you're going to spend $100-$200/year on service charges.
[8/24/2011] After completing a cross-country trip, I have a couple more observations. The tracking function worked very well, and friends could follow my progress on a web page. The batteries lasted more than a week of continuous daytime operation (but only Lithium batteries). The phone I ended up using with the device worked fairly reliably; about once a day, I had to pair the phone and the device again, but usually, they'd just connect.