I have recently modified my thinking about demoing prospective kayaks. I still think it is very, very important, but first impressions can be misleading. An initial demo should probably be for fit and comfort issues. A boats maneuverabilty, stability, speed and a few other issues can not be properly assessed in a single outing. I believe that most boats take some getting used to and it will take a while before a great variety of conditions can be experienced.
It is very important to figure out what you think you will be using the boat for. A boat that is hard to maneuver is not going to be very good in tight quarters and a boat that will turn on a dime is not going to be as easy to cruise as a boat that likes to track. The best solution is to have several boats, but to the extent that that is not an option, you must be even more careful about which boat you choose.