I have an ATA/100 device running on my B&W G3 with only the ATA/33 controller and it seems to be fine, if only too slow (just because it isn't operating at peek speeds).
You can buy either the SIIG or Sonnet ATA/133 cards, which will boost your performance. I noticed that the SIIG card worked fine when Jaguar came along, as noted by people here--the Sonnet card didn't.