I have the ergons like 'scaryseth' above, on a flat bar. I found I had to cut the flat bar down to like 550mm (21") from its original 600mm+ (23") for them to be really usable, otherwise the reach to the outside grips wasn't very comfortable. I'm a moderately big dude, too (6' tall, 210 lbs [down from 230], fairly broad shoulders).
I would think the ergons with the end grips (mine are the GP3) would be less useful on bars with swept back grips, as the ends wouldn't be parallel to the direction of travel, so using them would put your hands at an unnatural angle. My bike actually came with a mild riser bar with a slight sweep-back, it took me all of a week and a couple short rides before I scraped the straight bar off the smashed remains of my old bike, hah!