They have a demo version so you can see if your BT keyboard works with the application. The demo didn't work for me, though. I'm using the Apple BT keyboard, but after a Google search, it was suppose to work with it.
I've been looking/waiting for a way to use a BT keyboard on my iPhone, so I decided to pay for it, and it works as far as I can tell. Any box I've had selected lets me type through the BT keyboard.
When classes start up for me, I'm hoping to take notes through this, as taking my keyboard would be easier for my subway ride (and the Apple iPhone dock). I don't have an app for taking notes yet, but I'm definitely going to look into one now.