I have a "fruit cocktail" tree of stone fruits. It originally came with 5 kinds, a peach, an apricot, a nectarine, Stanley prune plum, and a Japanese plum of some kind. The nectarine graft died the year after I planted it, but the rest have thrived. Its a fun little tree, and amazingly productive. Getting kind of old, and I would definitely do more after it goes, I just enjoy the novelty of it -- freaks people out.
I think its a really great way for people with less space to get multiple types for pollination purposes, as well as diversity of fruit.
I planted two 5 on 1 apple trees two springs ago. Each has different types, one more summer varieties, one fall. I found them, of all places, at Meijer on sale for $18 each in June -- I wasn't expecting anything like that from a big box garden center, just random chance they would actually carry something worthwhile.
They have some buds this spring, barring this crazy weather might produce a handful of apples, but they're still kind of small.
We had a multi-graft apple when I was really little. It was a four on one or five on one. My idiot father decided to shape it up and prune it, and pruned all but 2 of the grafts right off. So, it became a 2 on 1 red and yellow delicious.
The only issue I see with these trees is that 1) make sure you don't do what my father did, and 2) because the vigor of the varieties varies, you have to be careful that no one is getting lost in there, blocked from the sun and covered up by a more vigorous variety near-by. Just be sure to prune carefully and it will work out fine.