My Kyook has been in my stable since early 1995. It was my first "Sea Kayak".
Mine is the Kyook Plus model that has both bulkheads and a rudder. The Kyook is 15 feet long and 25 inches wide. It has great initial stability and decent secondary stability. It is at home on flat water and pretty big waves and chop as well. I have surfed it plenty of times. It is not a fast boat by any means but will hold it's speed well. Tracking isn't bad but does require the rudder sometimes. I store my kayaks inside so the Kyook still looks brand new, even though it's 17 years old. Even the neoprene hatch undercovers are like new.
I'm 5' 8" tall and weigh 200 lbs and still have plenty of cockpit room. I could foam it in to fit like a glove like my Sirocco but I like the fact that I can move a bit for comfort and to fish or relax without bracing. Necky had some great plastic to still look as good as it does. I live in Western NY so all of our launch places are rock or concrete. No sandy beaches here. Thanks to the "old" Necky for making a great kayak!I purchased my Kyook - a blue 1996 with two bulkheads and a rudder - in the summer of 2005 and love it. It's the perfect boat for day or overnight trips on the rivers surrounding the Metro DC area, which are generally loaded with rocks (ouch w/ a composite boat) and don't demand a longer boat for covering ground or doing long open water crossings. That having been said, it's plenty long enough at 15' to handle bigger water. It's also stable enough to plow through class III rapids without much trouble.
The boat holds a ton of gear if packed properly, making it a workhorse on overnight trips. I carried the lion's share of gear for two people on a two-night trip on the Shenandoah without having to carry anything outside the bulkheads and the boat was still fairly nimble, particularly for such a wide (25") boat. With a regular-sized load, the boat is both fast and agile while still being very stable.
The boat tracks fairly well, occasionally exhibiting a tendency to pull to the paddler's left. I think this may be due to the oil-canning (dent) in the bottom. In deeper water, using the rudder fixes this problem. The rudder system is well-designed and easy to deploy and retract.
As for fit, I'm 6-1/195lbs and the boat fits nicely. My two complaints in that area: (1) my size 11.5 feet tend to hit the underside of the deck and (2) the seat is hard as a rock. I solved the latter with inflatable SeaLine seat and back pads, which can be velcroed on. They work like thermarest pads and can be easily adjusted for a custom fit.
I started looking for a bigger boat (in the 17-foot range), but am glad I settled on this length.