Depends on what they are made of. Here in Texas and probably elsewhere they are using the rubber from old tires which has been ground up and molded into shape and stained to look better. The playground surface areas are now being covered which is easier when a child falls plus no more muddy ground or gravel in that area.

I see them used in hardscapes on DIY network sometimes. They are made from old tires and are supposed to hold up longer than brick pavers.

pkponder, do they put a solid surface under them to keep them from sinking into the ground irregularly?

Oh yeah, foundation prep is the most important aspect to installing any pavers. Typically you'd want to dig 6 inches, lay landscape fabric then fill with crushed stone 4 inches deep and compact, level off with 1 inch of coarse sand and then place the pavers. Sweep sand (I hear polymeric is dandy because when you wet it, it forms a bond) into the cracks and your done! Guaranteed long lasting and professional looking patio! Lots of work but the absolute necessity if you want a level surface. I thinks decks sound easier!