I have a Griffin iTrip for my iPod. You wouldn't be able to use it with your iRiver, but one thing you should keep in mind when shopping is the interface used to change channels and the number of channels that the transmitter will broadcast too.
It seems like many of them out there will only broadcast to a handful of frequencies. The iTrip will pretty much do the entire FM band. The good thing is that you can usually find a clear (or clear enough) station to use. The only problem with the iTrip is that to change the station there are songs that you play that tell the transmitter which station to broadcast to. This isn't bad, except if your on a trip and need to change the station. You'll need to find a clear station first, then go find the track for that station to reset the transmitter. It's only a hassle on trips, around town I pretty much "set and forget."
Sound quality is decent. It sounds like a radio station. I would always choose a tape adapter over the FM transmitter when given the option. Just don't expect crystal clear sound.....especially coming from an MP3 player. But, it's good enough.
Hope that helps.