Used the HP-1 at the Pocono Nascar race with a race headset, after manually entering some driver's, track and official's frequencies using Arc Patrol.
The unit performed fairly well, however the audio is a bit too low. Turning off the AGC in audio options seemed to improve it, but an audio boost option would be better for high noise areas. Not sure if this can be done via a firmware update.
The other thing that was a bit of an issue was seeing the screen in bright daylight. Maxed out both contrast and backlight in the menu, but it still required shadowing the screen to read it. Be sure to turn on auto dim if you max these options: I didn't and I managed to drain the battery with about 25 laps left!
Oh yes, bring an extra set of batteries for the races!
All in all it was a positive experience; seeing the driver's or officials info on the screen was a nice improvement over trying to remember every car/driver number and scanner channel. The guy next to me using a regular handheld kept sneaking a peak over, so i guess he was as impressed as I was.