Good Idea Lacking Polish
Google Play has a music store, but the selection is limited compared to Amazon or iTunes. Google tries to make up for this with free music giveaways and specials, but those specials are usually price-matched by Amazon. In order to really make inroads in the music space, Google needs to have a larger collection of music or offer something different.
Getting Music to Google Play
In order to copy your music collection from your hard drive to Google Music cloud storage, you need to download an app to your hard drive. The app runs in the background and uploads music from any folder you specify, including your iTunes music folder. It does not unlock or upload music protected by DRM, which includes Audible audio books and some iTunes music files.
Here we find one of the first problems with Google Play Music. This uploading process takes hours, depending on the size of your music collection. I moved what I'd consider to be medium sized music collection from a fast Internet connection in a little over two hours. Granted, that was severa months ago, so the process may have changed. However, a large collection from a typical household Internet connection could easily take an entire day. This long startup process isn't inevitable. Spotify scans your music collection, and rather than uploading files, it simply knows you already own those files and gives you permission to use them offline. Apple does something very similar with iCloud.
The good news is that this happens in the background and that it only takes this long the first time you begin the process. Once you've uploaded your music collection, you can keep the Google Music uploader in place and use it to automatically upload any new music you purchase. I've used this to triple and quadruple-populate files in different music services by buying music from Amazon.com and downloading it to iTunes. You can also go in the other direction and download a backup of your music collection to put on iCloud or Amazon.
Second problem: Google can't tell if I'm uploading a duplicate. I'm not sure how many times I've uploaded duplicate and triplicate files. This isn't hard technology. Google should know what I have and whether or not it's the same as my other songs. Google can tell if a YouTube upload is a duplicate and recognize songs used in the background. Finding two copies of Take on Me seems like an easy challenge.
The Google Music Interface
Once you've set up your Google Music Player with files, you can use the service at music.google.com on any modern Web browser. It redirects to Google Play. You can also download an app for Android 2.2 and higher devices, which means most recent Android phones and tablets. You can also use the Web app at music.google.com on an iOS device or Kindle Fire if you're so inclined, but chances are you're just going to use the download trick to transfer your music.
Whenever you're connected to the Internet, you can stream music files from your entire collection. There are obviously times when you're not going to have a continuous connection, so your most frequently played files will be downloaded to your device for offline streaming. This works fairly well for the most part, although it doesn't work for devices dependent on the Web app only.
Playlists and Instant Mixes
When you upload music from iTunes, your playlists come with you. This is a huge time saver. You can create more playlists and create an "instant mix" of songs based around the selected song. This doesn't always work as well as it could. It's no Pandora. When I created instant mixes around songs, I'd get matches that included audio book chapters and other clear misses.
This is lacking. If your music has different relative volumes when you upload it, it's going to have different relative volumes when you play it back. It would be really nice to have some in-app controls to change the volume preferences for individual songs.
Google Music uses Last.fm to power its lyrics engine. This works well for the most part, but it means there's yet another service you must register for if you want this feature. You can also edit the lyrics and add them yourself if you find them somewhere else or just listen carefully and type quickly.
Google Music is late to the scene, and it lacks selection and polish. It's also a rather old-school approach to music for a cloud player. New services like Spotify and Pandora allow a rental or combination of rental and purchasing approach to music. Google Music is strictly about owning individual files.
Since it's free, it's no sacrifice to give Google Music a shot. I've found that I use it more often than Amazon Cloud Player in spite of purchasing my music through Amazon. Once they add a music store to the service, I may not feel the need to download my files first. However, the service still has some maturing to do, and all those other music services aren't resting on their laurels. Google has positioned this as part of a larger whole with Google Play, and that's good, because it doesn't feel like it should be a stand alone service. If Google were to acquire a service like Pandora or Spotify to add to the mix, they'd have a knockout music service. For now, they've got a decent enough iTunes alternative.