My opinion Cassette Adapter vs. Gruv-X FM Transmitter
The Gruv-X was a nice addition and I even wrote a little review of it in another thread here (http://ipodlounge.com/forums/showthread.php?s=&threadid=4096&highlight=gruvx) but I found that there was always a poor quality from poping static and I would have to constantly adjust the volume on the iPod so that the sound coming through the radio wasn't distorted.
another annoyance was that when I went traveling even just 30 miles I would have to change the frequency to find a station that was "clear". Now living in Denver the radio frequencies a near full and I expect drifting and bleed in but I found a better alternative.
sticking with the old Cassette Adapter worked great for me! the sound quality is superb. It may not be as good as direct connect using Aux cables but there is no popping and no adjusting the volume on the iPod. you can adjust the volume on the iPod and there is no distortion, it just gets louder and still clear. another added advantage over the Gruv-X was I no longer have to worry about turning it off. when you turn off the vehical the Cassette adapter also stops.
I know this may not work for everyone. there are alot of people out there that have CD ony decks and no place to put the iPod close enough to the deck and in that case a FM transmitter may work for you.
I just wanted to let the iPod crowd know that if you have the choice between the iTrip, Gruv-X or any other FM transmitter and the Cassette Adapter, you will be much happier with the plain old Cassette Adapter

