Depends what I'm using it for.
To gauge the ability of a video card to perform a particular feature or function, it's okay. To compare video cards alone (pretty much forgetting the rest of the system) in a completely synthetic scenario, then it's great. To work out how fast your machine will run UT2003/Doom III etc, it's a load of rubbish.
Assuming you mean 3DMark 2003 that is, although the same rules more or less apply to 3DMark2001.