In my ever-changing search for the best and easiest way to listen to music for free while on-the-go, I've tried several methods.
The first was to simply use a separate mp3 player. This allowed for the most control over the music I'm listening to, at the cost of obtaining the music and manually synching it. There was also the issue of creating playlists, which is something I'm not good at. Another downside was that while listening to the mp3 player, I was unable to hear phone calls/texts/etc. However without having a smartphone, this was my only option.
Once I got a smartphone, I discovered I was able to listen to FM radio stations through my phone. This allowed me to be able to hear incoming calls/texts/notifications while listening to music. Since it was a radio station I was exposed to new music that I wouldn't have in my collection. However, not only did I have to put up with commercials, but also static and noise due to poor reception or interference.
The next thing I tried was Slacker Radio. I chose them because their app came pre-installed on my phone and was unremoveable. I had used Last.FM in the past, but their mobile app was not free, and that was a deal-breaker. Slacker radio initially seemed to be a good choice. I was able to choose genres and let them pick a playlist for me. The service was free, and the commercials were few and far-between. At least initially. With each update to the application, they removed more and more features from the free version, and increased the frequency of commercials. On top of that there were blatant bugs that were never addressed, like songs skipping randomly, or the app crashing.
Then Google Music came along. "This is Google," I thought. "Surely their product will be flawless, free, and feature-filled."
For starters, you have to upload your personal music collection to their servers, so no discovering new music. Not to mention it takes ages for anyone with a decent music collection (although that is more of a complaint against my ISP and the poor infrastructure in the US). Not a deal-breaker, although with each corrupted or mislabeled song it becomes a bigger issue.
The app's UI is decent enough, although it takes longer than I'd like to start playing music. An instant-shuffle of an artist, album, or genre should be more easily accessible. Another thing is that when a notification plays on the phone, the music dims, but does not pause. This may not seem like much, but even Slacker radio paused the song then automatically resumed it when the notification sound stopped. The instant playlist option is nice, though it heavily favors the artist of the first song, and the playlist is too short. Ideally the playlist would continue to add new songs as needed.
The bugs are not disastrous, but are still annoying. Songs will sometimes repeat without the repeat option being on, and without updating the display to indicate that it is being repeated. It also often takes more than one try to get a song/playlist/etc to start playing.
Overall, Google Music lacks the polish and quality of Google's other services. After some improvement, it could be a very good replacement for carrying around a separate music player, however it's unlikely that it'll be able to replace music streaming services.
Right now its biggest upside to me is that it's free and I don't need to create yet another account solely to stream music.