I was tracking a few friends at Boston on the BAA official app that used the timing mats for official splits and times. In addition, I was tracking my friend Ricky Campbell on RaceJoy.
The apps worked pretty well together! The BAA app gave 5K chip splits and the RaceJoy gave GPS Mile Splits. The finish time is only 12 seconds off between the GPS time and the Chip Time. This is probably a combination of figuring out the start time as well as the normal error of ~5 seconds due to GPS splits.
It also looked like they had good cell coverage in Boston for the day – all alerts went out well on both the BAA app and the RaceJoy app.