I use the iRock FM transmitter for listening to my MD in the car. For most songs I have no problem with it, but for some other songs the sounds quality is not terrible, but bad enough that it makes me depressed it's like that.
My boyfriend has an iPod and uses a cassette adapter. His music sounds great and I'm attributing that difference between mine and his, to the fact that he has an amplifier for his iPod.
His amp plugs into the cigar adapter, has a stereo input from his iPod, and a stereo output to his cassette adapter.
But just to be sure, I'm going to nab his amp tonight and try it on my car with my MD player. Hopefully it'll work for my MD player, I didn't get a chance to take a good look at it.
EDIT:
If that doesn't work, I just remembered my housemate made his own amplifier from an Altoids tin, all with parts that he bought from Radio Shack. I'll probrably go borrow his Altoids amp tonight, if my bf's amp doesnt work for my NetMD, and see if the amp helps the sound quality.