Ultimate Frisbee is an exciting fast-paced game, quite difficult to photograph.
I found that a shutter speed of at least 1/1250 is required to freeze the action.
Good autofocus is absolutely essential and when the light is low a sharp large-aperture lens is useful.

My standard equipment is the Canon EOS 1DmkIII and Canon EF 70-200mm-f2.8L IS USM which I use handheld. This system has awesome AF and the zoom is useful. However this lens is not fully sharp at f2.8 so I stop it down to f4, where it's very sharp. Normally I position myself at the middle of the field (at the side) to reach both endzones.
When the light gets really low I use a Canon EF135mm/f2L USM.
Another lens I tried and which has given the very best pictures is the Canon EF300mm/f2.8L IS USM which is just razor sharp even at f2.8.
I also tried the Canon EF100-400L/f4.5-5.6 IS USM which gives decent results in good light but less good in low light and not fully sharp at f5.6. Very useful zoom range though.

When shooting I turn off IS and use manual exposure settings. I haven't found any metering system which works and gives consistent results. By using manual settings I can fine tune the exposure to avoid the white shirts to get blown out. When the light is consistent it's easy to find the best exposure, but when the light is changing it requires some fast adjustments.