Why? Mostly for fun. I really like it so far though, it was really simple and easy to do. I've had alot of issues with RSS where I would have to escape a ton of stuff just to get it to validate. So far with JSON Feed, I have had no issues.
It's also a very nice feed to look at. You can almost read my blog entirely by just looking at the JSON Feed of it.
I also updated my Github repo for lambblog so you can see how I made the implementation: https://github.com/lambdan/lambblog/blob/master/jsonfeed.php
In other news, I've been busy with other stuff so I've been kind of dead on Twitter and here. That should hopefully be over soon though.