I have two of them and I used to just love them for a while. Then I tried the original, sold in Sweden as "The Russian bed of nails", which is all plastic so it's easier to wash and is inflatable so it's easier to carry with you when you travel. It also has 4000 "nails" instead of 6000, which makes it a little more painful in the beginning but more effective.
However, I went from those to one that is sold as "The Swedish bed of nails" that is a simple pice of relatively hard plastic (actually it's four pieces that you put together) that has 600 fairly long but not so sharp plastic nails. THis one is now my favorite although it was pretty painful to lie down on the first couple of times.
They do have different effects. The two first ones are nice to lie on, the third one is more of a thorough massage and afterwards you might feel a little sluggish.
They all cost about the same and way, way too much for what they are made of. But then again, that's the reason why I say "Hi" to my boss every morning, inmy most pleasent voice. He gives me this salary that I really love.