As far as desktop video cards, omega drivers void the warranty. I dont know if it applies to laptops necessarily however.
I think Omega overclocks slightly and also changes the setting from "quality" to performance, and thats what makes you get the extra few points in benchmarks, but its really not that significant anyways. Getting 200-300 more points in 3dmark05 only means you're getting 2-3 more fps in a game and if you're already crackin 60fps, you wont feel any difference if you would be getting 62 fps with omegas.