Rudy,
Shipping to North America could be pricy, due to the size. If you're really interested, it's best to contact Oolaalaa directly and work something out with them. In due time, there might be a distribution point in North America though.
Erin,
They are actually quite comfortable, provided you sit in a size that's suitable for your length. They come in 6 sizes: XS, S, M, L, XL and XXL, shown here are XS, L and XXL. The L is meant for kids or short people. If I sit in it, it's comfortable, but getting up is difficult because it's so low. Kind of like sitting on a chair that's too low for you. The XXL is great for me. Plus, since it forms itself to your body, you're not limited to one particular way of sitting.
Ben,
It took a long time, but that's partly due to the fact that printing needs to be done in fairly large runs. Up until now, we've been doing test prints, but even those would result in several balls, so they would sell those and we'd only start working on the next test once the stock had run out.
Rafal,
I suggested doing the moon as well. The XXL and XS have roughly the same ratio of diameter as the Earth and the moon...