I can sum it up to: Team fortress 2 meets DoTA (warcraft 3 custom game) Here is a link to the forums. And a link to my assault guide.
Edit: I've appealed to the UBER development team to give me some more gift passes, there seems to be at least some interest.
Assault is my favorite class, I've written an in-depth guide for how to rock the assault. I have 2x uber streaks with the assault under my belt, and I'm level 78 so I feel like I have a fair amount of experience. http://www.mondaynightcombat.com/forums/viewtopic.php?f=31&t=7441&sid=68b23992bdf3c59fee983fccd042ed73
Remember to be playing with game sounds on and listen for the assassin cloak noise. If assassins are giving you trouble upgrade your jet pack and charge before your bomb and you'll notice a significant increase in your surviveability. Check out the endorsement recommendations I make, I have almost no problem with assassins at this point in my MNC 'career'.
I've had a couple of problems with using stations in the last couple of patches, sometimes it only accepts inputs if I'm standing in the right spot. Keep your back to a wall and your ears open. learn to spam jump and shoot to buy yourself a bit more time and never use your grenade launcher against them, unless they're far enough away and you shouldn't have too much trouble with them. Lastly, don't forget, as an assault you're going to be worse at killing bots than Gunners and tanks, but you can take advantage of their failings with your mobility quite easily if you're smart about it.
Assaults in my opinion are more about outsmarting your opponent than being more skilled, although some will probably argue that those two are synonymous but with MNC I disagree.
There isnt an demo out there that I know of. The studio is currently doing an engine overhaul, so its likely that they will run a sale when that is done in a month or so.
On the actually running it front, this thread may help.