I've been signed up for Google Music since the closed beta. I've yet to try it out though. I have over 1,200 individual songs though. The thought of uploading them all is daunting to say the least. Besides, I don't know that I'd ever really use the service. I can't think of any situation where I would personally rather stream my music from Google than just listen to it locally on one of the many capable devices I have.
It's been hinted at, but I haven't seen it spelled right out... Does the service replace your uploaded tunes with higher quality files from their library? I have a lot of old Napster files that weren't too great back then, and have only gotten worse as times goes on. Sadly, only about half of my collection is lossless, so that might be an appeal if true.