I used the n900's FM transmitter to stream ccMixter podcasts over edge/3g on a trip from San Diego to LA monday.

Was a rental car (wagon-ish) with the antenna on the back roof.

Was only able to get good FM signal with the device clipped to the headliner in the front (in my old e51 case), or held up by passengers in the back.

Quality was OK most of the trip, most of the issues we ran into were bandwidth related on the cell side in spotty areas. We did have to change the FM frequency once along the way after getting overpowered by a LA station.

Overall it worked fairly well. I was disappointed however that the FM transmitter will not function while USB is plugged in (a yellow message alert appears notifying you of such)