I have four Yo-yo's, and I absolutely adore them. They are as passive to my other fish as they could possibly be. Occaisionally they will pick at each other a little bit, but it never amounts to anything. I've never seen any of my Yo-yo's do anything even remotely resembling harmful intent toward any of my other fish. They are active and seem very curious. If a group of fish gather close together, you can bet one of the Yo-yo's will swim right through the middle of them, looking side to side as he goes, as if to say "Is something interesting going on here?" Sometimes they'll get right behind another fish and swim directly behind it for a few moments like they're playing "follow the leader", and then they'll break off and investigate something else. It's rather amusing. My other fish are not afraid of or skittish around the Yo-yo's whatsoever. They act just like the Yo-yo's are their own kind.
There are some conflicting statements/opinions on the web about their disposition in a community, so I don't know whether this is due to variance in temperment or just misinformation. The guy at my LFS told me once (before I owned any myself) that Yo-yo's would antagonize tetras to death, and should only go with fish that will fight off bullies. My experience with my four could not be more different.
Hope that helps!
pendragon!