I've been running Subsonic for a while now, and it's been amazing. Not having to regularly sync my old Droid is great. Mobile streaming has been fantastic, and the web interface is even a bit nicer. Setting up accounts for friends is awesome, and I'm constantly making them sample my changing tastes, in an effort to quell my rants. I've been putting Subsonic through it's paces, that's for sure. Allowing last.fm scrobbling on a user account basis is rad.
I signed up for a Google Music [beta] invite a week back, and got my invite today. I just set it up. It's only mildly different from Subsonic, though I guess the main benefit is that my computer doesn't have to be up and running as the server. Still, I have a feeling I'll stick with Subsonic. But I might as well hop on this Google project and help them test it, and of course get in on the ground floor ;)