I love Google Music Beta for the same reason I love(d) my 160GB iPod classic. I simply hate picking and choosing what music I want on my device. It's tedious, and I never have everything I'm going to want at any given moment. I love thinking of a song or having someone else mention a song, and chances are very good that I've got it, and got it on me right at that moment.
It's rare that I have stuttering or problems with playback, though when I do hit a deadzone, I sometimes need to wait a few minutes to get back in a signal area. If you live in a place where you frequently have 1x instead of 3G, then don't bother, 1x can't handle it. I believe Google streams your songs at the actual bitrate, so if you have high quality MP3s, expect to use a lot of bandwidth streaming. The up side is it sounds very good (better than Pandora, but not as good as an actual CD obviously).