they can be great but you need to know your F's first.
F1 is 50/50
f2 is 25/75
then F3 F4

first you take a serval(or other exotic cat) and breed to domestic F1 then breed the offspring to a domestic F2 untill F4 or F5 and they can brred back to themselves. Up untill a certain point the males are sterile so they can breed back to themselves before then(not sure it that part is the same for all exotic cat hybrids). Kind of like mules but it's just the males(or maybe just usualy the males not positive).

Anyway you want an F4 or later. They still look cool have a lot of the plusses of servals but are considered domestic at that point and wont be spraying everything or shouldn't while ALL full blood exotics will. You can always work your way up later if you decide to go into the real full blood exotic cats.

Savannahs are part serval and bengals are part asian Leopard Cat. Bengals come in a little more variety.

found a good over view page and has some listed I didn't know about.
http://www.savannahcatbreed.com/breeds.shtml