Hey Craig,
I've paddled a Mark 1 (deep seat) for the last 2 years, and is my first "serious" boat since using a spec ski. When I first got it, I spent a fair amount of time practicing my remount, and I wouldn't classify it as a beginners boat! Of course, after 2 years, it feels rock solid stable
The comments above are a fair reflection of the Mark 1's performance, and my own experiences. When the conditions are flat, it can be tough keeping the speed up; although on the downwind it can really get going. I've done some big downwinds (DBN World Cup 2007...), and I've been impressed the way it picks up on the runs. It's stable, predictable, and quick enough to have fun.
As the other posts have said, it's an older design, and dates back to +/-1994 when Mark Lewin (Custom Kayak's owner) used it for Molokai.
The one thing they haven't said though, is that it's a really tough boat. I've given it a fair amount of abuse, and it's come through alright.
I'm 100% biased towards the Mark 1! It also helps that the service I get from Custom Kayaks is exceptional.