Pretty amazing game all things considered. It's been around for a year now, so the price is lower than release, and I'm sure you can get it for next to nothing from either a Humble Bundle, or waiting on a Steam sale.
As you can see from previews, it's a top down sneaking/stealing game. You've got eight characters total, each with their own main ability, and a less noticeable side ability. Example, if going by the preview, the Lookout is the expert at Recon, which means when she's sneaking/stationary, everyone gets red icons on the map indicating where guards/civilians are, even when not in vision.
Her secondary power is that she opens ladders/vents 3 times as fast as anyone else. The main power of each class is the draw, but often you'll find the secondary power is incredibly useful as well.
There are a rich amount of stories, though given that this game is a year old now, online co-op can be somewhat hit or miss. There are still players, but you might not be able to get a game every time you look.
Another frustration is that later missions become really very difficult for a solo player to beat. It's always completely possible, though sometimes it requires some extra work. This really is a game that benefits from co-op.
As people have mentioned, getting seen or alerting guards will not be the end of you, and in fact is required sometimes to move guards out of position. That said, later missions in say the 2nd story or the epilogue will punish you quickly for not being stealthy.
Finally, for a small little indie title about pixilated thieves, there is actually a pretty good story being told here. It presents itself as story's being told by one of the thieves in a police interview. As such, the 2nd story is the same heists, but completely changed around, and a step up in difficulty as it's another of the crooks recollection of the tale.
It's up to you to puzzle out the whole truth of what really happened here. The reveal comes in the final mission of story two, but it's easy to miss what happened if you aren't expecting it.
I continue to play this game and highly recommend it for the price it is now (15$, and certainly recommend it on sale)