Their official installer is nasty shit, seriously. Don't use it. (OK, it is your system, but don't say nobody warned you.) If you feel comfortable poking around figuring out why X won't load, use their installer. If you want it packaged in a semi-sane manner that probably won't destroy your system* use the distribution specific packages.
(*This isn't to say that FGLRX won't randomly lock up your system forcing you to hold in the power button, but that's on a bad month, like this month's 11.9 release. Usually.)