We've found ourselves plenty impressed with Google Play Music of late, but one area of concern has been with mobile data usage. Looks like that's being addressed in the most recent update (version 5.0.1052J, for those of you keeping track), along with some other improvements.
Of the greatest interest to us is the addition of "new settings for mobile network bandwidth usage." Taking a quick look at the updated app, you have now have a "Mobile networks stream quality" section in the settings with three options -- low, normal and high, and there's definitely a noticeable difference between the best and worst quality.
As carriers keep pushing us away from unlimited data plans -- and they've been pushing a lot harder over the past year or so -- that's become a cause for concern when you're away from the relative comfort and safety of Wifi. Google says this new version also reduces overall data usage, which is a good thing.