I had (have) one and it was never much good. Very very low power indeed, as in, it's ok if you keep it in the ashtray right under the radio, but no real transmission distance at all. Also the battery life is dire - expect about an hour from a pair of AAAs before the fuzzy signal means you have to replace them (although I believe the actual iTrip is powered by the iPod itself?). Also the one I have is on frequencies between 88.1 and 88.9 and this does get affected by radio stations sometimes.
Oh, and they're illegal to use in the UK
Great idea, just a shame they don't really seem to deliver.