The question isn't so much if the adapters are any good, but rather, where do you live? If you live in filthy crowded-frickin' Los Angeles area, where almost every frequency is taken, then your iPod signal is going to sound good, mostly. Sometimes you'll have to switch to another "empty" frequency, depending on where you're driving in the LA basin.
If you live in po-dunk Idaho, where you have a few radio stations, then you'll most likely be very happy with the results. Just think about how your radio sounds, at it's best. That's how good an iPod radio transmitter can sound.
Up until now, i've been using a Belkin transmitter, throughout LA and OC area, and I have to switch frequencies sometimes, otherwise some radio stations MAY impose on it. But for the most part, for me, it's a decent experience. Not amazing, but I consider my '90 Maxima incomplete without my iPod.