I'm using RaceChrono, and like it. I also picked up a Qstarz 818xt from Amazon (~$90 or so). The stock GPS in most phones samples at 1Hz. With the external GPS you get 5Hz or 10Hz (depending on how you configured it).
It was a little bit of a pain to get the GPS to work with my phone, but I've used it now with both a Galaxy Nexus and a Nexus 4 without problems. Might try the Nexus 7 next time.
Sample data from the built-in GPS:
And from the external GPS:
