I asked for a tryout for the Google Music Beta and just recently received my invite. I really like the basic idea of the system, i.e., storing music on "the cloud" rather than on your laptop or on some external hard drive or a stack of CDs filled with mp3s in your basement, and then going through the transfer process with a new computer, etc. Google Music also doesn't require syncing a phone and computer and music player as one has to do when using Itunes on an iPod or Iphone and on a computer.
Another nice feature is that they offer a nice selection of free tunes as well, if desired.
However, the initial description isn't completely forthright, and feels like a bait and switch. The earliest description had led me to believe that I'd simply be able to store my music as mp3s and download them to any device I wanted and use any software that I wanted. However, it seems that that's not the case. Rather, one can download to other devices but they have to be Android devices, and, unsurprisingly, I guess, there's a limit to the number of devices. One can also download to a computer, i.e., laptop or desktop, but it seems that on your laptop only the Google musicplayer can be used to play the downloaded music. So, it seems that Google Music commits users to Google software or a Google OS. As such, insofar as one doesn't want to make a lifelong commitment to using Google products, it offers no solution at all as a storage medium, I'll still have to store all my MP3s on a hard drive somewhere. I guess they're not being evil, but they're no better than Apple's nearly useless proprietary music software and formatting.
(oh, and one other odd glitch, gtalk won't display the current song title when one is playing music with the Google music player.)