As long as you're not doing any demanding 3D (games), you're fine. But sadly for some titles, the 30x speed difference makes the free drivers a no-go. I learned this the hard way when trying to play Skyrim and grudgingly had to switch to fglrx.
It would be great if changing between the two drivers didn't involve as much config file changes and a reboot (module unload doesn't seem to be enough). That way I'd use the free drivers most of the time, and only switch to catalyst when the need to swing a beautifully-rendered magical axe arises.