Personaly, I much prefer 1 design. I would much rather sail in a 6 boat A cat fleet then in a 20 boat portsmouth fleet. I am far more interested in the sailing then the social aspects of regattas. However, I also (belatedly) realized that if you are going to train and practice, you better have a boat that you enjoy sailing. Some previus boats I have owned have been fun to race because of the competition but have not been very enjoyable for me to sail. I found myself hating the boat even as I enjoyed the regattas. I prefer responsive, high tech boats that I enjoy sailing. I can go out on any given day by myself on the A cat and have a blast sailing.
Fortunately I have an outlet and that is the A cat in the summer and the DN iceyacht in the winter. If I lived elsewhere, I would probably be screwed. I honestly don't know what I would do if I moved to an area that I couldn't race one design.

Sail Fast,
Sail Light
Eric Anderson
A class US 28
DN US 5193