I have the HW2 mic and yes it's a bit pricey but worth every penny.
This mic actually has 2-way communication built into it so the camera operator and the subject can talk to each other over the greater distances. This is nice to have when you're 100 feet away from each other... you don't have to yell.
It's not actually a surround mic but rather monophonic. What happens is that the on-board mic continues to operate producing surround sound while the audio for the center channel is REPLACED with what is coming from the HW2. The end result on playback is the ambient surround sound from immediately around the cam comes from your front/rear speakers and the everything from the HW2 is heard from the dialog (center) speaker. It makes for some pretty amazing surround sound.
The audio quality from the mic itself is pretty bloody good. The only downside is that it eats batteries. The mic takes a single AAA battery and you can kill one in about 1.5 hours