I've virtually given up with balls! We had a good one that was clear and all in one piece, but it wasn't really big enough for our large Syrian. It's ok when they're smaller. I then got the Savic 25cm one (in fact I tried a load of 25cm ones and sent them back - the Savic was the best of them all) and don't like using it - I've seen his toenails sticking through the vent slots sometimes and it isn't completely smooth inside, with projecting bits.
This is the first one we had - it's very good, all in one piece rather than those ones that come in two halves that clip together, and it's clear.
Amazon.com : Living World Plastic Hamster Exercise Ball with Stand, 6-3/5-Inch : Pet Habitat Decor : Pet Supplies
It might do for a baby or small Syrian. I still keep a ball as it's a good way of transporting them from a to b (ie you can carry them in it safely without them jumping out of something or falling while you carry them.
Better to go for a playpen area really and put a wheel in it, in my opinion. In Germany they don't use balls at all.