I can't speak to Ultimate Jet, but I worked for a competitor 5 years ago. My company went bankrupt and is no longer around. The last post mentions delays and I have to disagree with my experience. I am finally flying private again and while I have lots of delays now, I did not while flying the Dornier. 30 passenger charter is nothing like flying a King Air or Citation. We flew a lot of sports teams (Nascar, NCAA, etc.) and they generally know their schedule pretty well in advance. For us, once something was on the schedule, it was probably going to happen and was probably going to be pretty close to on time.
All that said, the DJET is a great plane. It is slow at cruise, but the first time you see 7000 ft per minute in a climb, you won't care anymore. If you can get on with Ultimate (or any other operator), you will love the plane.