Avoid if possible. Outputs to X11 (uses shared memory extension), with no hardware acceleration at all. Supports (MMX/3DNow/SSE accelerated, but still slow) software scaling, use the options -fs -zoom. Most cards have hardware scaling support, use the -vo xv output for them, or -vo xmga for Matrox cards.
The problem is that most cards' driver doesn't support hardware acceleration on the second head/TV. In those cases, you see green/blue colored window instead of the movie. This is where this driver comes in handy, but you need powerful CPU to use software scaling. Don't use the SDL driver's software output+scaler, it has worse image quality!
Software scaling is very slow, you better try changing video modes instead. It's very simple. See the DGA section's modelines, and insert them into your XF86Config.
If you have XFree86 4.x.x: use the -vm option. It will change to a resolution your movie fits in. If it doesn't:
With XFree86 3.x.x: you have to cycle through available resolutions with the Ctrl+Alt+Keypad + and Ctrl+Alt+Keypad - keys.
If you can't find the modes you inserted, browse XFree86's output. Some drivers can't use low pixelclocks that are needed for low resolution video modes.