Savannahs (like many other wild animal crossbreeds) are illegal in a lot of places, but for reasons that aren't entirely clear. There's a fear that they're dangerous (servals, of course, are) though there aren't any known cases of Savannahs attacking people. The following passage gives you some of the flavor of the controversy:
That's not the way State Senator Carl L. Marcellino of Syosset, N.Y., sees it. Mr. Marcellino, the Senate sponsor of the state's exotic pet law, objects to the Savannah cat as something alien to the animal universe."Breeders are creating animals for commercial purposes that would never exist in the natural world," he said. "These hybrid species are threats to the environment and potentially to the families who think they are buying a family pet and could be purchasing a wild animal."
The Savannah cat has caused a stir about what makes an acceptable pet even among the largest and best known cat enthusiast groups.
"I'm told they're very loving, but I'm not sure I believe it," said Carol Barbee, the president of the American Cat Fanciers Association, which does not recognize the Savannah in its official registry. "We do not want to support designer breeds for the fad pet market."
Some Savannah owners are fighting for their rights with ammunition from another group, the International Cat Association, which does recognize the Savannah as a breed.
"They are the sweetest most gorgeous things you've ever seen," said Leslie Bowers, the association's business manager. Dr. Carolyn McDaniel, a consultant with the Cornell Feline Health Center, said that while Savannahs are popular across the country, she has noticed that they have become particularly alluring to city dwellers. "It's amazing to me that apartment dwellers are frequently the owners of these large semiwild cats," she said.
"I think they're beautiful," Dr. McDaniel added, but "I'll watch them on the nature channel."
In the face of all this talk about how unnatural Savannahs are, it's easy to forget that modern domestic animals don't really exist in nature either. I don't know too much about the direct ancestors of domestic cats, but dogs, descend from wolves, which you wouldn't want to keep as any kind of pet. Yet, there are plenty of dogs with very sweet dispositions (and of course others which are vicious). This isn't to say that Savannahs aren't dangerous--I've heard some negative things about wolf hybrids, for instance--but that's a question to be answered empirically. The mere fact that they're a hybrid doesn't really tell you much at all.

The fact that all of these breeders beind over backwards to reassure people that these custom cats are the sweetest animal ever seems slightly suspicious somehow. As the former owner of a cranky siamese mix, even a normal sized cat can be a handful, much less a 2x normal sized!
The big question to ask is about having these hybrids released into the wild. Eventaully, somebody is gonna get hold of one of these things and decide they no longer want it, run out to the country, and leave it there. Then what?
Does the Senator wish to ban steaks, modern corn, rice, and wheat? Seems like such a ban would have to apply to all of these as well as the Savannahs unless it came riddled with so many exceptions as to make it useless in any case.