I had cornish rexes (cousins to devons) so they are bascially the same cats. Can answer all your questions (plus I know devon owners).
Activity level - very high, love to play and run and jump, and climb high.
Affectionate? - very people oriented. Will help you out in all things you need help with or don't need help with. Will be your lap buddy and your sleeping buddy - UNDER the covers.
They are very smart and clever - can learn to open doors, cupboards, etc.
You cannot free feed rexes - they are 24/7 chow hounds and if left to eat all they want will become overweight very fast! Devon females should be 5-6 lbs, males 6-7 when full grown. So you don't want them on kitten food any longer then 5 months old!
Grooming - practically none - mostly hand grooming. Good devons have wavy coats and all you need is a flea comb and use every so often. They do tend to drop coat more then the cornish do, so sometimes they have bare chests/bellies. An occasional bath will not hurt them - maybe once a month or so. White rexes will need bathing more often to keep them white.