I bought Airbox Clarity a couple of days ago. I'm a new PPL and have only tried it in air once. Really liked the CAA chart with visual airspace warnings and relative positions to towns/airfields etc.
I was told that GPS is good as a "backup" to your dead reckoning, probably because I just took the step from student to pilot, but after 5 minutes usage the map and plog went to to the back seat and the gps became my main device for navgation and plog/chart took the role of backup
The clarity is relatively cheap for what you get, but I can see that chart upgrades (other countries than uk) will add to the cost. The battery life is fairly short but can be boosted by an external battery booster (not necessarely an expensive airbox one).
Typing in letters for user defined waypoints etc. is a bit crappy. It's touch screen but it doesn't overlay a keybord. You have to step through the whole alphabet to get to the characters you want. hopefully this fiddly process will change in a later software version.