We just got back from Belly Shack and in a word: YUM. Bold favors, nice people, cool vibe... and they're running like it's well past their first day. (Probably didn't hurt that Bill Kim was expediting!)
Here's what we tried:

Super tender
Korean BBQ Kogi with a bit of fresh kimchi and sliced triangles of bread that reminded us of very good pita
Asian Meatball "sammich" with vermicelli noodles and mint. Lots of lemongrass and herbal notes, great bread
Roasted Brussels Sprouts with plenty of spicy chorizo - totally surprising how much we loved this dish
Dessert was the softserve (the only dessert they serve, but in fantastic flavors) - we tried the
Vietnamese cinnamon and salted caramel. Best salted caramel ever! Sorry, didn't remember to take a photo until it was half eaten.
Much like at Urban Belly, it's serve yourself water and silverware, take a number to your table and wait for a lovely person to deliver your food as it's ready. I also got an Izzy peach soda, and they have a couple of green teas available by can. Doesn't look like they are preparing beverages of any kind.
Incidentally, Steve Dolinsky was at Belly Shack at the same time as we were, and it looks like he ate the same things we did (and more!), if his Twitter posts are an indication. The difference between us is that I didn't get a tour of the kitchen at the end of the meal, as he did.
As I posted on Twitter while we were there, my hubby overheard that in two hours, they had served 55 diners and 11 huge takeout orders. They didn't seem rushed, though the kitchen was buzzing. At times there was a small line, maybe as many as 7 people deep, but many times no line at all.
HIGHLY recommended. Don't be afraid to check it out right away!
"Whatever you are, be a good one." -Abraham Lincoln