They're so cheap it's hardly worth not fitting one.
I got one for my birthday last summer, by request
Something like £20 off ebay, came with a free PAYG sim on Lyca mobile, which has a handy feature of being able to top it up online, and set it to top up automatically when it gets below a certain credit limit, so there's no chance of it running out of credit.
For an extra £10ish, you can get a 12V hard-wire kit to allow it to charge up from the car battery. Mine is wired to the ignition live so as not to flatten the battery. You can set it up in either of 2 ways. You can have it constantly updating its location at a pre-set time period (every 60 secs for instance), which you can follow on a map. I think that's more for 'Big Brother' style company bosses keeping tabs on their van drivers though. The downside of that setup is that it costs more! It uses 3G data rather than text messages, so it only sends a small data 'packet' with each ping, but it adds up.
Mine is set to just respond to a phonecall. Dial the number of the sim card and it'll hang up, and send a text back with co-ordinates, time, battery condition, speed (if moving) and a google maps link to show a pin on a map. It'll only reply to you if you've already set up your mobile number as a 'user'. You program it with a series of text commands, one of which you use to add up to 5 authorised users - any of which can call the tracker and it'll reply. If any non-authorised number tries to call it (random cold call for example) it won't text them, so there's no risk of a random caller getting sent a text with the exact location of something potentially valuable! This way is cheaper, as so long as you don't call it, it won't be using up any credit. The only time it'll contact you out of the blue is if the battery gets to 5%. So I know if I've not used the car often enough when it texts me to say the battery's low!
You can also program it with lots of other automated alerts. You can set a 'geo-fence' so it'll text you with a warning if it goes outside of a specified radius of a specified co-ordinate, or you can set a max speed alert so it messages you if it goes over 70mph, for example.
In reality, I played with it a lot in the first couple of weeks, but now I just forget about it. It only costs me 10p a month or something for when it occasionally texts me to say the battery is low.
Arguably it's not exactly 'value added' at the moment, but much like an air-bag in a modern car, you hope you'll never need it, but the one day you do, you'll be glad it's there!