Jason,
I have successfully uploaded my 15,000 song collection to Google Music, as well as my numerous WMP Playlists. Once this is done (properly) by telling the Google Music Manager app (on your PC) to sync with you WMP collection, all syncing to Google Music will occur automatically in the background.
To answer you other concern, regarding limited cell coverage... I agree, and am happy to advise that Google Music provides a solution. Google Music for Android allows you to designate any music in your cloud collection to also be copied down to your device for offline listening at any time. I find the best way to work with this is designate several active/current playlists you are enjoying, and designate them for offline listening on your phone. Whenever you add or remove tracks within those designated playlists, a simple sync of Google Music will update everything again.
I was skeptical of Google Music at first, and it will take a LONG time to initially upload a large music collection to the cloud. However, once that's done, and you dial-in your playlists and off-line selections, you'll begin wondering how you lived without this musical convenience for so long.
Good Luck & Happy New Year!