My new hamster, Sherlock, loves sitting in his. He is a climber though so it isn't surprising.
He grapples his way to the top of the cage and climbs round the gaps in the arms that attach the sputnik to the top of the cage, hangs off the entrances, hoists himself around, it's quite funny to watch. He'll go in and out throughout the night.
He used to chew the top of the bars he could get to by half monkey-barring, but since then I've stuck a little wooden gnaw toy for him. The sputnik and that together make chewing a little bit more interesting for him so he doesn't attack the bars so often now. Can be useful as well as fun.
I think it'll depend on your pet if they use it or not, if they like climbing they'll probably like sputniks. They're really easy to attach on and off, so if they don't use it, it's no worry to get off.
If they don't seem interested in it, hide some treats in it? I put a little bedding in mine and hide two or three little tit-bits for Sherlock to sniff out.
Thinking of putting some sand in there soon though, see how that goes.
