I've had it on a few flights and it was a pain in the bum at the time - that was a few years ago though, and hopefully it has been improved.
The automatic voice taxiing on the ground was so loud it would make it impossible to hear ATC transmissions, and there was a lot of "say again" requried.
The database was also rarely up to date with the global airport network, so if you diverted to certain airfields you would have to inhibit the EGPWS to shut it up (And yes, that was the approved method!). It would also warn you that you were approaching runways that had been decomissioned a few years prior. Definately a formal inhibit switch required for those issues. Being told you are entering a runway when you are not just leads to complacency.
The concept is a good one, however there were definately significant bugs that needed solving - and I'm not sure how you solve the one of having your line up and takeoff clearances blocked by "Approaching Rwy....". I hope they've fixed a lot of the bugs and that it becomes more common - but there are definately airport layouts that work well, and ones that it couldn't cope with.