I wouldn't recommend those- they have very poor ventilation. Plus, rats already have very bad eyesight... being in a ball like that makes them pretty much blind. I tried a ball with two of my girls, and it freaked them out pretty bad... they wouldn't move or anything- all they really did in there was peed.
You could let them have free-roam over your bed or couch... that's what I did with my girls until I really trusted them to have free roam of my entire bedroom.
If that isn't enough space, I would recommend either rat-proofing an area of your house (like a walk in closet, bedroom, or hallway that can be covered off easily). If that isn't a possibility, then you can always use the bathroom- just make sure to take anything that you don't want them to get into off of the floor first!
madeittothemoon's got it covered. Chances are if you try one of those balls all your rattie will do is look at you like you've lost your mind. I got one for free a couple years ago and tried it out for kicks. Very uneventful lol
Rats are too smart for them anyway. Beyond that, they're not worth the expensive, for reasons already mentioned. But if you already have one, they do make great hanging nests. Keep the top off, and hang 'em up.
i put a small version in the cage and yesterday when pookie was making the whole where the lid goes bigger it sounded like she was choking so i took everything plastic out of the cage. now i am sad because i don't have many toys in the cage just two hammocks and a circle thingy i mad out of cloth that they can climb on