From what I hear, and based on past experience with kayaks and a lot of other water craft. I think they will be very stable, but also very dangerous if there is any breeze at all. The will be prone to wind drift as they don't have as much under the water as a conventional kayak.
I would say they are suitable for flat inland waters and rivers where there is very little wind influence. I know that they are also used on white water due to their stability.
http://kanuschule.ch/funyak/index.php
I haven't used an inflatable kayak myself though.