I think I'll need to adjust my phone tariff. I was curious about data usage, after a day of using Google Music on the go. On average I seem to use 2 - 10mb a day, and about 300mb a month. On Tuesday, after several hours of Google Music I'd used 260mb!
I'll be looking for an unlimited plan as soon as I can change my Three contract.
I'm just coming to terms with the impact of using Google Music on data usage :-) If my calculations are correct then it will take a little over 7 hours of continuous play to use up 1GB of data....
(That assumes that you are listening to music with a constant bit-rate of 320 kbit/s. I don't know how Google Music adjusts bit-rate for the bandwidth that it actually finds to be available.)