I have a Linex FM modulator, only the non-usb powered version. (uses 4 AA batteries).
Soundquality is ok, and its very stable, during my drive last year on holiday (1200 km) I never had to re-scan the tuner, so the PLL does its work very fine.
Only my unit has 8 pre-programmed frequencies, using dip switches, but thats not a real issue. Can't compare the soundquality to a regular radiostation, since i don't use the unit anymore.
I was planning to make it battery-powered myself using a voltage regulator, but I decided to remove my HU at all and connect the carpc directly to an amp.