The Polarstar gearbox is a Electro-Pneumatic Airsoft Rifle, or EPAR. It uses the external high pressure air rig that many of us tanaka/kjw users have. It sounds awesome and performs extremely well. The "gearbox" will be adjustable via the internal electronics. You can adjust the FPS, RPS, air volume, and even set it to whatever burst settings you'd like It has been stress tested at 70 rounds per second at 400 FPS, but it will be limited to I believe 35 RPS at 450 FPS. However, there will be other nozzles available for purchase that will allow you to increase your FPS past 550. Basically, for the $450 it costs, you can go from 30RPS 400 FPS "AEG" type gun to a Semi-auto 550+ FPS DMR in only a few minutes.
And yes, I'm getting one. I've been trying to decide on externals for it, but I've never owned an m4. I was really interested in either the Masada, or the Scar. Something you don't see everyday.