Turtles have always fascinated me in the wild, and every so often I will run across one when I'm out strolling near a pond. I've never kept one as a pet however, even though I once kidnapped a wild turtle for a few minutes to show my nephew when he was a toddler. I'm not sure I would know enough about them to keep one as a pet, because they do have a particular diet that I'm not sure fish food would fully accommodate.
Plus, if given enough space they will grow to their full size and some get pretty large. They are long-lived animals so if you get tired of caring for them when they get bigger, that's an issue too. It's sort of like when the goldfish gets too big for the bowl and you don't have an aquarium... what do you do... give him away to someone who is better equipped or do you drop him in your local fishing hole? Some folks do make the wrong decision there, which is why goldfish are now found just about everywhere, including the great lakes.