Just as with toys for children, before you get a toy for your cat you should
check for yourself whether there are any loose parts that can be pulled off
and swallowed, dyes that could make a cat sick, or any other hazards.  The
company selling them sure isn't going to tell you what's not safe about
their product and *unlike* toys for children there is no oversight
organization looking out for cats' welfare.

Speaking of cat-related products, here is a link to an inexpensive
alternative to a cat tree:
http://www.abcdistributing.com/store/0505F1/34.cfm?vc=0000000000

And here is a *really* cute bird feeder:
http://www.abcdistributing.com/store/0505F1/92.cfm?vc=0000000000

Hugs,

CatNipped

You need to take into consideration the particular cat's
idiosyncracies.

My cat, Oscar, has had one of those plastic balls for years, and she's
never chewed on it.  It's bounced down hard stairs many times without
any damage.  So, for Oscar, they're safe.

Anything with string, though, I cannot allow Oscar to have.  Her cat
tree came with a toy on a string.  As soon as I attached the toy, she
began to chew on the string and would have eaten it if I hadn't
removed the thing entirely.

For your cat, hard plastic balls sound like a bad idea.