Generally cassette adaptors work well. The biggest problems is the poor channel separation and poor frequency response. Otherwise they work great and are simple to use.
FM transmitters generally work poorly due to the design of the transmitter and the transmit power limitation. These limitations result in poor sound quality and channel separation. Low transmit power means powerful radio stations tend to interfere with your little FM transmitter. If you are looking for one, make sure you get one that uses PLL synthesizer. This helps to generate a stable frequency for your transmitter to lock in to a frequency that you transmit. Often one of the biggest problems with cheap FM transmitters is frequency drift. After a while the frequency drifts away to the frequency that you set initially.
I have tried many many FM transmitters before in the past. None of the portable ones like Belkin Tunecast etc etc. work well. Only broadcast quality stuff works well since they are better designed and not limited by cost. The only portable one which worked well for me was the LineX (you can't buy this here). It is the only portable FM transmitter with a PLL synthesizer built-in. It works pretty well. I still have it and was using this before I got the aux-in enabled. Since then it's been lying in my drawer gathering dust. It has also been modified to work off the cigarette lighter adaptor.
If anyone wants to give it a home and can put it to good use, make me an offer.
