I saw the trailer for Nation Red and was thinking..."What...the hell is this?! "
So, I'm installing the demo and just what to hear what you guys think about it.
Thanks.

I highly recommend this game. And have in fact bought it twice -- which should say a lot about it's quality. The developers are exceptionally good at listening to community and working with them in regards to improving the game -- as such, get the Steam version if you're going to buy it. It's cheaper and is readily being patched with new modes, characters and so forth.

Defined patched...as in new content or bug fixes?

I played the Steam demo and enjoyed, it was pretty cool.

Both.
The bug fixes don't really need mentioning but as for new content:
New character
New control scheme (such as you now have the ability to lock weapons, set how dodging works)
On-the-fly adjustable difficulty levels added (this was actually a direct result of an email I sent the devs =))
New power ups
Upcoming new content:
Practice mode (which allows you to set game parameters)
Steam achievements
Co-op multiplayer
Ability to mod the game
Dusk/nighttime levels
And more =)

Well, after I download this Area 51 installer I'll give it a test run.

Sounds rather interesting, but... no. Not for me. Reminds me of that Flash game, Boxhead.

I rather enjoyed the demo, what it did allow felt like the last-stand/survivor modes in games like ShadowGrounds and Alien/Zombie-Shooter. But with a little more variety.
It's the type of game that has potential for expansion (and DLC :S). I'm keeping an eye on it to see what their commitment is, as a number of small/indie titles come on steam, promise features and they don't end up happening.
But I'll probably but it next time I need to spend time blasting away at something and I'm bored with my current game collection of that genre.

Personal opinion, I downloaded the demo, title screen looked EPIC, music was EPIC, intro movie was EPIC, title was EPIC....and then I was playing Zombie Shooter/Alien Shooter/Smash TV.
Yuck. Whoever designed the aesthetics for that game really had their talent wasted. I guess if a game trying to capitalize on Left 4 Dead is your idea of fun, and you don't own any of the *.Shooter titles, it's a decent purchase. Can't beat the price, and the graphics are suprisingly solid for the genre.

Trust me, their commitment is very very solid. Just as an example, I can now name 2 things that have or are being implemented into the game based off of my own feedback to the developers. The difficulty level slider, and the upcomming change in health drop rate (where lower levels will see less and higher levels more).
The next update comes out this week and then another will be released in October which introduces co-op modes and, most likely, modding tools.
Except that this game is far more fun than L4D has ever been. If anything was cashing in one anything, it was L4D cashing in on all the zombie games that came before it, such as the Killing Field mods, etc.
It also plays differently to the Alien/Zombie Shooter games, so that comparison isn't really good enough either. Although it is similar to pre-existing games. But then, the same can be said for pretty much *any* game...

I hope the new patch improves it, because the first time I gave the game a go I though it was rubbish. Zombies hit you from a distance, all maps are exactly the same, leveling system is stupid, character movement is clunky...

When I first looked at it, I saw that it was singleplayer and was put off by it. But those options I highlighted have pretty much just made me want to buy it now.

Yup, those features should be rolling out sometime in October. You can see a list of most of the upcoming features here:
http://forums.steampowered.com/forums/showpost.php?p=11300774&postcount=1

My first impression is that it is a fun game. Pretty solid. But it still needs a bit of work. Judging by the upcoming updates, it will be a great game.