I've had the Roadster myself for about a week and a half now, and the MyTouch 4G just a week beyond that. I love having a no-wires setup for both calling and music playback, especially since no aux adapters were ever made for my car's headunit.
The Roadster has a nice little speaker on it for voice communications (don't fret, call audio can still go through FM audio), a decent FM transmitter*, good battery life, and easy to use buttons. I specifically like that you can manually adjust the broadcast frequency across the whole FM band, rather than be married to a handful of presets that may or may not be crowded in your area. A nice benefit, also, is that the Roadster comes with a Micro USB CLA, so I didn't have to buy one for my MT4G. =)
* - It took me a few days to find an optimal freq to use in my area, but once I did, the signal was quite strong)
The Roadster is meant to be used with Motorola's MotoSpeak app which, in my opinion, sucks. I've never been overly impressed by voice recognition in general (let me be the first to point out, that may mean I'm
) but MotoSpeak is resoundingly lacking, both in quality, and capability, when compared to the Genius button.
As for sound quality, your happiness will be dependent on your taste in music, and your ear's attention to detail. The audio sounds compressed; while capable of thumping lows and decently sharp highs, the product seems unable to do both at the same time. Any composition that has rumbling bass and crisp highs in its mix will lose one, or both, depending on the tune. (I can't think of the term for this phenomenon... fairly certain its not dynamic range or frequency response, but is obviously related to both.)
To be honest & fair, though, I'm not sure there is an FM solution that will NOT compress audio.
For what its worth - those are my two cents. Congrats, you drug me out of lurking. =)