I bought Warhammer 40,000: Kill Team one of the last times it was on sale, and it's a decent enough twin-stick shooter for someone who isn't necessarily a Warhammer fan. It took me around 5 hours I think to get through, and I'll probably play through it again with one of the other classes.
The demo for Wreckateer was pretty good, but with my backlog growing a demo really needs to pull me in for me to make a purchase. If it ever gets cheaper I'll probably pick it up.
I'll try the demo for You Don't Know Jack, but I wish Metro 2033 had one too. If I want both I'd have to buy points, which I'd like to avoid.... The Darksiders demo was fun when I played it a while back, but I'd rather buy a used disc copy for half the price.
Side note: I added Metacritic scores for a few more games in the OP.