Depends on the puffer, I guess...
I have a figure 8, and at first he wouldn't accept feeder guppies at all. I work at a LFS, so I've tried all different sizes. He still wouldn't eat them, until one day I brought home some very tiny guppy fry. Well, he inhaled those! We didn't try again until about a month ago. I was going on vacation, so I brought him home a few small guppies. I figured, I'd feed him his regular food before I left and again when I got home -- but if he got really hungry, the guppies would be there for him. So... I released the guppies in the tank...and BAM... he bit one in half and ate it immediately! I was a very proud mommy.

And all the guppies were GONE when I got back.
So I guess... it took a little training for him to recognize them as food. I don't really like to feed him guppies, though. They are so over crowded at the stores, I'm afraid of bacteria and whatnot. They aren't my (or his) first choice.
Foods he loves -- snails (they are like candy to him!), krill, all types and varieties of shrimp (live, fresh, frozen, freeze dried, pelleted), tropical fish flakes, lobster (when we have lobster, we'll save a piece of a leg for him). The sinking shrimp pellets are good because they are nice and hard when you first drop them in.
I house my own snails and shrimp in other tanks. But what I'd really like to do is setup his tank with live shrimp so that he can do some of his own hunting. I just need to finish arranging the tank decor first. Right now there aren't enough hiding spots, so it would be pointless. Kind of like fishing with corn.