Where do you live? My (limited) experience has been that they can be OK away from big cities, but that finding a free frequency in urban areas is really tough. They put out enough power to work when that's all there is on the frequency, but even tiny amounts of external signal on the same frequency make it go to crap.
Most of my driving is in and around DC, so it's no good there. The modulator works great, though. No antenna involved, so no interference! Well worth the work to install.
