Mine had the cracks too. When I went to move them from the stock rims to my new rims, the tire shop found one had a bubble on the interior sidewall (couldn't see it with it mounted on the car). So it had to be replaced. Then when they took the other one off, they tore the sidewall of the tire getting it off. So they bought me a new tire. They said it was their fault, but I'm betting the tire was weak from dry rot or ozone whatever-you-call it.

I would replace them asap, especially if you plan on doing any high speed driving. I put Yokohamas on the fronts of mine. I like them. I can't recommend the Sumitomos I have on the back.