I've been using the Google Music Beta for about 3 months now, and I can't say I've seen or heard of a better option to enjoy all (or at least up to 20,000) of my songs without lugging a storage device large enough to hold all the files. The elimination of the need to have my iPod or mp3-player on me in order to enjoy my music is a paradigm example of the direction in which the current cloud-based technology is advancing. Prior to trying the Music beta, I had been expecting to have to hold out for a flash-memory-based media player in the 120-160GB capacity range to drop to a more affordable price before I'd be able to have the bulk of my music library available on the go, without relying on the questionable life-span integrity of many hard-drive-based devices.
Since the web-app is obviously still in 'beta,' you have to request an invitation from Google via the website and have an Android device in order to use the mobile app, but if you can get the invite and have a 'droid, you can use it free of charge.
Music is uploaded via the Music Manager application (installed on the computer with the music) which is now available for 32 and 64-bit versions of Windows, Mac OS X, and Linux (.deb and .rpm), and can be accessed from the main Google Music page via the "Upload music" link once you have logged into your account. Also included (at the time of this writing) are offers to add additional music to your collection (also free of charge) from Google's database.
Since the music is stored in the cloud, you can upload additional music from other computers (with the Music Manager application linked to your account) on top of the other songs already there. Access to your collection is limited to 8 devices (other than computers), but if you attempt to connect a 9th, it gives you the option to revoke access from another device and grant it to the new one, so you aren't left hanging. You can also log in to your Google Music account and play your music via your web browser on any number of computers as well.
The only potential snag with the mobile device playback, is that if your data connection is weak, it may take a bit of buffering-time to get started, although no more than your average YouTube video, from my experience. Also, since your files are streaming over your data connection (similar to streaming Pandora), it will eat up your monthly data allotment more quickly than many other types of mobile web activity, unless you have an unlimited data plan as I do.
Overall, I am very impressed with the Music Beta from Google, and eagerly anticipate the release candidate. It took less than a week for me to get my invitation when I requested one, but I'm not sure of their current degree of availability. Give it a shot if it seems like something that may interest you; even if you don't have an Android phone, the browser availability is rather convenient.
