I didn't get a chance to play as much tonight, since I was getting a few adventure games installed.
A few observations though:
I really dig that you can upgrade each weapon to a "double shot" without using more energy per shot.
Some people might fuss that using the same ammo for everything is lame. I kind of like it though.
Running around everywhere will get you chewed up.
The enemies will come after you if you try to retreat.
Strafing is an absolute must, to reduce incoming fire.
THe big beasties get pretty scary. They can soak up a lot of hits.
You can actually buy keys in the intex style computer, but its wicked expensive.
Looking up and down is nice, though its a bit awkward as its all by keyboard.
You only get a password every 5 levels. Holy shit.