Ok so I bought a Belkin Tunecast FM 2 a while back and to be honest it sucked. I made a youtube video with a way to make it less suckish but it still sucked.
The Belkin cost me $25~ plus a couple hours to mod it. Now the problem is depending on the weather the thing works great. However 90% of the time you will end up with static bad audio quality low bass and you have to mess with it on long trips to stay off of other radio stations.
On the flip side I went and got a Cassette to 3.5mm headphone jack adapter and hooked it up and it worked great! [Aside from a few minor things]
Some of the issues with this is that if you turn the volume up on either the radio or the ipod to much the audio will get distorted but once you find that sweet spot you are good to go! Bass response is great when I hooked it up with my iPhone. Other than my car attempting to switch sides on the tape a few times in my 1 and a half hour trip it worked great.
So to sum it up… If you are looking to get something that can play your ipod in your car and have a cassette player. Go with that for around $10 [from walmart]
If your car does not have a cassette player and has a CD player I suggest going the more expensive route and getting a ipod adapter for your head unit because to be honest the FM transmitters suck major ass.