Yes, I do have some experience using it. The Sentry was installed on a recent ferry flight aircraft and when switched on in European airspace it showed it is almost completely useless here. The big features are UAT based and there ain't no service here.
The best cost-benefit ratio I currently see in Europe is a raspberry pi based STRATUX with three antennas and the current STRATUX-EU distribution from b3nno on github - plenty of options and gives you all US coverage plus the local european specialities like Pi3, Flarm, OGN etc. when needed.
Be aware when entering European airspace on a California mindset that you are about to enter air over paleo-digital deserts.