Really cool! I just love the simplicity of it: has no drawbacks compared to regular textarea, but has lots of benefits. you basically improved textarea, by a lot!

I also made a similar thing a while ago called contextarea.com, maybe, I should add overtype!

Having to use a monospaced font is a pretty big drawback. To me, it means I wouldn't use this for a product that wasn't intended for a techie programmer audience.

Not that it isn't a really cool project! I'm only saying it has clear drawbacks.