I'm on Windows Phone 8 with an AT&T Lumia 920 and just activated a free trial of Spotify. I have a 2GB data plan. I played around with the settings this morning, and I activated offline mode, not sure exactly what it would do. I also turned on Stream with high quality, sync with high quality, and sync over 3G. I used it in my car ride to work and I would say the car ride was about 25 minutes, and I went through about 7-9 songs. I didn't listen to most of the songs all the way and skipped a couple times. Once I got to work, I got a text message saying I used 65% of my data. My cycle starts on the 3rd of every month and I'm usually connected to Wi-Fi so I don't always use a lot of data. I turned off the offline mode because I'm assuming that it probably used a lot of data. I then got consecutive text messages that I have used 90%, and now 100% of my data so I'm going to end up paying extra this month on my cell phone bill.
Did I do something bad by activating offline mode or skipping through a couple songs? I'm fine without using offline mode but I tend to be a song skipper so that habit won't change. I really like the app but won't be willing to buy extra data to use it along with the Spotify Premium fee.