Turtle is tasty, but you SHOULD NOT eat the big ones. They accumulate too much toxins and chemicals in the meat because they've lived so long. a 30-40 lb snapper might be 100 years old!

Also, you should let the big ones go if you want to have turtles around for awhile. Snappers don't breed untill theyve reached 15-20 years of age. by killing the big ones and throwing back the smaller(tastier!) ones, you can really do a number on the breeding stock. (plus, i have this thing about killing anything older than i am)
good luck!
moose