Already own Defense Grid from a prior sale and I'll add my voice to the chorus of folks recommending it. I'm not a big tower defense guy, but this is a great entry in that genre and there's a lot of play value here. I like the story/narration and the interesting, incredibly detailed environments and units.
Ran through demos for just about everything else last night. Darwinia+ was alright, but the demo didn't really convince me that it was worth 800 points on sale. Given its "classic" status, you might need to have had some prior experience with it to really appreciate it.
I really expected to like P.B. Winterbottom more than I did. It's fun and I like the play mechanics and production design, but I could see it getting frustrating and boring over time in the very same way Braid did for me. Not sure if I'll bite on this one or not before the sale is up.
Chime clicked with me immediately. LOVE the music and the gameplay is easy to get into (though I don't fully understand what's going on yet). Doesn't hurt that it's dirt cheap and gives you a 50-point achievement just for picking it up. Bought that one immediately.
I played the Rocket Riot and Poker Smash demos a while back, during a previous sale, and again found them just okay. I have way too many GREAT XBLA games backlogged to kid myself that I am going to play the middling ones, even at sale prices. It's especially hard to spend the points with A-list titles like Puzzle Quest 2, Monkey Island 2: SE, and DeathSpank on deck, plus DLC for Toy Soldiers and Defense Grid. And that's just in the next two weeks.