I did find service to be spotty at times, but I think this is more a function of the carriers and the terrain.

Service was spotty in some areas but the biggest issue I had was even in bigger city's like Frankfurt, Berlin, Stuttgart etc

I found the SIM worked the best in the UK, fairly well in Ireland, and about as well or better in Belgium and Holland with 3G data (only place it said 4G was in the UK).However, my phone frequently lost services for a few minutes at a time, only to have coverage restored a few minutes later.