Are there any aftermarket systems that come close to the functionality of SYNC that would work in the 05-09 Mustang? I'm specifically looking for voice commands and a usb port that can play mp3s from a memory stick...

 

The only aftermarket headunit, that I can think of is the new Pioneer F900BT/F90BT. It has total voice command for both phone and song info, and it has a USB connection. Only issues I have heard is the slow response times to imput functions. Its why I haven't gone and upgraded my AVIC-D3. That thing works flawlessly, but over on AVIC411.com, there are threads about the "bugs" in the new F-Series radios!

 

I have the F700BT that I just installed. Avic just released the 2.0 firmware and it works great for me so far (although I never used the old firmware). Plays songs from USB, iPod, or the SD card slot in the front. I think for voice command music playing you need a iPod though. The 2.0 firmware seems much faster/more responsive than previous reports I read, I'd recommend giving it a try at a store or something (make sure they've upgraded firmware) and see if it suits you.