Good enough for what? Depends what you want it for.
I have a g550 which works fine, but don't expect any kind of decent 3d performance. 3d is very weak but that isn't what the g550 is for.
The drivers are included with X, though you'll want the proprietary mga_hal module if you want to use the fancier features like dual-head (haven't tried that yet). You can find mga_hal on matrox's website, along with a configuration tool which I seem to recall worked fine (it's been some time since I used it).
I'm currently using a Radeon 9800 pro which takes ages (3-5 seconds) to switch between X and the console (and also to switch resolutions in X). I've had a Geforce which also took several seconds to switch. The g550 switches very quickly.
In all, it's an excellent card, as long as you don't need high 3d performance.