I too used to use Simplify Music. Orb is pretty good but it's more manual-ish, like...you just browse the folders and listen to music manually. Plus it's pretty expensive for what it does (it sucks for video streaming, Air Video beats everything hands down for that)
I was looking around for alternatives but then I remembered I've heard lots of rumors about music streaming, that Apple was also going to allow users to stream their iTunes libraries to their iOS devices. Granted, these are just rumors, but I figured I would wait until the September keynote where they introduce the new iPods, which is also rumored to be when they announce the iTunes cloud movement.
If I don't hear anything about this there, then I'll continue looking for alternatives. I would also wish Air Video came out with a music streaming app, since they do streaming media the best.
Haven't used it much but it works pretty good. I don't care to leave my MacBook Pro constantly on when I'm not at home. It will occasionally tell me my Mac is offline when it's still on. I guess it may be the desktop software that times out.
I also vouch for StreamToMe. Along with music, it will transcode and stream your movies in just about any format, something Simplify would not do. Also, the developer is quick to address issues and support emails, if you have problems.