I've got the basic 16GB WIFI only version with a dedicated bluetooth GPS unit (can be used on iPhone, Laptops, iPad etc) - google GNS 5870 receiver
I've just upgraded from the "free" version to the pro version as I've ordered some other charts which aren't available to download (i.e. the southern ed37 chart) which came on CD today. Appears better functionality on this one, better customisation for the tracking view etc.
I've never had stablility problems though - I think when I first got it, it bummed out on me but the battery was low and it shut down a little after it so put it down to that
Just for info, the GPS receiver was showing accuracy to within 4yds in the air, and on the ground at the flight desk was 10yds.