I go to every Dexter day that I can.
As mentioned above, the majority of the cars are bad ass fast, and a few of them are pro racers in real race cars. You should expect to see about 60% Porsche, and the remaining cars are pretty mint as well.
As one of the slower cars (1.43~1.44 minutes/lap in my 1993 Corrado 1.8T) I can say that in order to have fun on the track, you will have to have had some decent experience at Mosport, and you definately have to watch your mirrors.
If you are comfortable driving a track that has such potrntial for disaster, then you will surely have a good time. Dexter is strict, but he runs one of the tightest ships around. I've been to tonns of lapping days run by different organizers, and invariably you end up getting screwed out of your run times...
My favourite thing about Dexter days (or any lapping day for that matter) is passing Porsche/Audi/BMW drivers with oversized egos. Most people can accept being passed, but there are a few that HATE being passed by a VW.