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.