I wish I had my Xbox 360 with me. It's still in Las Vegas at the moment. I'd totally sign up for this. I remember really enjoying the Spec Ops games back on the PS1 (oh the memories) and loved how they were solid titles at budget prices at the time.
If I could give mine to anyone I would. Really, you guys aren't missing much. The controls are stiff and frustrating if you're used to the Gears controls. Really, the only thing separating this from any other 3d person shooter is the sand element.
I was going to post about this last week and completely forgot... I believe they are still accepting testers on a wave basis. When the last wave was sent out (last week or so) I also received an email telling me to redownload the beta because they had re-released the beta.
I was accepted Day 1 and I don't recall an exact size, but I believe the file was 700 MBs or so. The new one is listed at 10xx MBs. In the email they said they added a couple game modes and three more maps to what was previously included.