IMHO such third party drivers are useless, or rather pointless.
What they offer exclusively are usually mere features that can be tweaked with a good third party drivers tweaking utility such as ATi Tray Tools, replacing Catalyst Control Center, or nVidia's equivalent that replaces the official control panels. Mr. Joe everyone usually won't bother looking for third party drivers tweaking tools and will often use official stuff, which is just decent for basic settings changes.
For example, Omega drivers will (pure example here) claim improvements/optimizations for Anisotropic-Filtering over the official Catalyst drivers. Well, the trick here is that the official drivers are already capable of that very optimization, the only thing that Omega does is to activate it either through file editing (the very dll files from the drivers, or other such files related to them) and/or tweak the registry to enable that optimization, which normally isn't adjustable at all in the official Catalyst Control Center. So with ATi Tray Tools you can just download the official drivers, only the "display" drivers, and then install ATT. And that tool will allow you to change/disable/enabled such features and optimizations freely through simple options that you have to select or unselect.
There might be some exceptions here and there but I never really saw any differences with such third party drivers. Anyone right in their mind about driver settings tweaking will just download the official ones (or beta ones from the GPU company) and instead download a nice tweaking tool, rather than an already modified driver that might just cause twice the problems you'd get from official beta drivers.