I've had one for a couple of years and just bought a second one since I'm brewing more lately. I upload to brewstat.us so I can track the progress which is nice if I'm traveling or something.
Recently started brewing under pressure in a keg, and it helps a lot with that since I can't see what is going on inside. The bluetooth range is not good when it is in the keg, but I just go right next to it with my phone and it gets a reading. Recently also setup a Raspberry Pi to read the bluetooth and I can set that right on the keg to receive data continuously. The TiltPi is very easy to setup, they have a nice SW image that you can install in ten minutes.
I always use hydrometer or refractometer for the official gravity reading, but the tilt graph definitely lets you see how the attentuation is progressing, and easy to see if you bump up the temperature one degree if that affected the fermentation.
Also, Beersmith recently added support to import the tilt data, so there it's nice to keep a record of fermentation gravity and temp there. I'll be using that from now on.