I don't quite agree with Eric and Bob.
I usually sail Hobie 16s but last summer I went to an 18HT regatta an while I did have a lot of fun (thank you Peter) I left wondering; did I have any more fun than I usually do racing my 16? No way! I was going faster but the name of the game was the same, trying to go a little faster than the guy next to you. The only difference was the 18HT cost about 30k by the time it hit the water and my 16 costs about 7k. That and the fact that I can race the Hobie 16 in 30 boat fleets all the time.
Different strokes for different folks I guess.

BTW I found those class membership numbers from 2003
Hobie 16 was about 550 (hard to tell as its a % of the Hobie class)
A class was 131 members
Formula 18 was 45 members