I used mine in Phoenix, San Diego, LA, Las Vegas, San Jose, and San Francisco and it worked just fine. Worked better than my iTrip that i ended up giving away. Occasionally, as with any fm transmitter, you'd have to find a better station to use if in the larger cities, but that was rare.
Gotta say though, I now LOVE my cheap alpine deck with iPod inputs and menu functionality. I could never go back to FM transmitters.