Which Hobie 21 is it, there is a Hobie 21 sport cruiser and a Hobie 21 SE. The Hobie 21SE is a sweet ride, but is a ton of boat. I have two friends with them and they were meant to go fast. They are a bit of a pain to set up as you have to expand them each time because of the 10' beam. Common problem's I've heard of with these boats is that parts haven't been made for a very long time and are hard to come by. Also the sockets that the wings slip into tend to get cracks which cause leaks. No big deal, you just have to fix them. These boats can be single handed in the right conditions (light air), but both of my friends need help setting them up.
I sailed with one friend in San Diego bay this past April and had a blast, we then took made the crossing from San Pedro over to Catalina Island.
I also crewed for my buddy with a Hobie 21SE in a Division 3 regatta in Sausalito this June and we recorded a top speed of 32.2 miles per hour on his GPS.
I would imagine that on the great lakes a Hobie 21 would be awesome! I'd also consider a Hobie 18, also out of production but Hobie still makes parts for them and they're much more common than the H21.
Take care, and good luck finding that new boat!