I had a GPS one (Get MD Log or something). I would never use it in the race, but would use it in practice and qualifying. I would set myself a target time (usually the lap record) and work at getting as close to that as possible. It is nice to know you are moving in the right direction whilst you are out on track (have I mentioned I had the most pole positions last year?!!
).
I think without timing whilst I was on track it would be easy to over-ride. This way, I could ride smoothly and calmly and see the results were still good (I find if things are flowing well and you are going quickly it can often feel slow, so this way I had an actual measure).