I ordered the Griffin iTrip for the iPod this last summer. It had been on backorder for months and finally came in this week. I am disappointed in this product.
The two advantages that the iTrip offers over other similar devices like the iRock is that (1) the iTrip draws its power from the iPod and therefore needs no batteries and (2) the iTrip can be tuned to any vacant FM station rather than being limited to two or three choices.
When I installed the software, however, the "playlist" that defines the available stations did not copy, and thus there is no way to tune the device to a desired FM station. I am stuck with the default 87.9. More importantly, however, the iTrip offers very poor range. The iRock that I bought this summer will transmit to my car radio and to an FM radio in my basement without any problem. The iTrip, by contrast, seems to have a range of no more than about 12 feet, and it will not transmit through walls or floors.
This has been a long wait for a disappointing product.