One would assume that since they got accepted into YC that they had a pretty good hypothesis of what their target market could be. But it's not clear from their website what it is (and it doesn't look like it's an IDE).

The OT technology that powers stuff like this is interesting but is being rapidly commoditized by JS libraries (check out the awesome ShareJS http://sharejs.org/ for an example). So what makes these guys interesting hopefully isn't the tech itself, but the market they want to bring it to.

How would you use it in an IDE? For pair programming?

Certainly can't imagine it would replace source control - it would be incredibly frustrating having other devs break your codebase all the time, you'd be debugging all their mistakes as well as yours, and as a team we'd be disincentives from making any big changes since it would halt anyone else's work while the change is being worked on.

There's a reason we create abstractions like commits and branches, and don't all just work from a share drive.

So their market is a free IDE?