I've had a little experience with different ways of handling my music collection, and this is a brief comparison.
First, and mostly, I've used iTunes and an iPod (really an iPhone). This, I think, is still the best, but that may be because I'm so used to it. I've put some effort into setting it up to work for me, so it does what I want. I nearly always listen through the mobile device, and I like that information about that listening is synced back to iTunes. I use that metadata to control the contents of a smart playlist that I listen to a lot.
Then I got an Android, which doesn't come with a fat 32G memory like the iPhone, so I gave the Amazon Cloud player a try. It works, but in general my network connectivity isn't good enough to use it consistently. I'm most commonly listening on a moving train, so spotty coverage hurts bad. It also doesn't support ratings and smart playlists. On the plus side, the uploader works well, and I got to live beyond my 8G limit when the network was around.
Then I got a 32G memory chip for the Android, and I started using iSyncr to sync to iTunes. This works well! It updates iTunes with changes to ratings and play data, so it gets updates to my smart playlist just like the iPod. My main complaint is that podcasts are disorganized (or rather, organized into one big playlist). I liked the way the iPod gave me a list of subscriptions with an episode count for each, and I could drill down to listen to what I was in the mood for.
I tried DoubleTwist briefly, but changes to ratings on the phone never made it back to iTunes.
I recently got an invitation to the Google Music beta, and I've given it a brief try. The uploader just shoved up everything from iTunes, so I have podcasts mixed in with my music. Ratings are thumbs up/thumbs down, which I could maybe work around, but I wonder why 3/5 stars became a thumbs down. It's also missing a smart playlist feature, but it's not as bad as the cloud player when the network is bad. The Music player on the Android maintains some local cache, so it can keep playing offline. I haven't played with this feature a lot, but it looks as if I could be as happy with it as with local storage most of the time.
Ultimately, I think the Google Music service is going to be my next home. It makes sense to put music there, access it from anywhere, and stop carrying gigabytes of music around everywhere. If I could have the kind of smart playlist I use every day in iTunes, I'd move to it now and shop for another way to do podcasts.