Our DS LOVES his... it's the only thing that keeps him quiet when he's teething & fussy. We got plenty of replacement bags, for when one gets so messy we don't feel like bothering with cleanup.
So far, he's had fresh or frozen (peeled) chunks of mango, apple, peach, banana, nectarine. We just keep a bag in the freezer, and toss in a few chunks of whatever I'm making into baby food, before I puree it. (I buy some of our fruits frozen, as our DS is a handful and fresh fruit frequently went bad before I could find time to prepare it.) Pretty much anything you're already feeding will work.