I also bought an ipad 2 wifi only and needed a GPS. The Bad Elf is very good but I was afraid of damaging both the GPS and the ipad connector in the cockpit. Therefore, I've been looking for a bluetooth GPS. The GNS5870 has apparently a tricky power switch that would easily turn on in the bag. I finally decided to buy a bluetooth XGPS150 and I'm happy with it. There's one bug with this unit : it loses signal above 10'000m, but the firmware update will be available within the next 10 days.
HTH