I've just ordered it with the lap timer bit.
The lap timer is the main reason for getting it, I got fed up with practicing at tracks I was soon to be racing at with no way of telling if I was improving my times or not. And I don't care what anyone say's, using a stop watch is inconvenient (another person required) and not as accurate.
The temp sensor should be another way of telling if your gearing is too far in one direction, and you'll be able to tell before
it goes bang
But it's just as easy to pull over and have someone temp your car from the pit lane
That's one of the problems, you need someone else!
The telemetry box looks about the same size as Spektrum's micro receiver.
Can also tell you your maximum speed, again usefull to see if your getting the most from your gearing.
Also tells you the signal strength from the tranny to the car and it can measure your battery voltage.
If your not interested in these things, then it's not worth it. But I'd get it for the lap timer alone for the reasons mentioned above.