I have a flatbar with triathlon aerobars and I can assure you it is wonderfull
They aren't cheap though for sturdy ones and there are a whole lot of different types outthere so choose wisely, since most of them are made for triathlon-geometry bikes and will be too long for normal hybrids.
Take a look at the picture-thread on pages 45-46 ... I posted some pictures there of my set-up. The effect of the tri-bars was immediately noticeable when going against the wind or downhill.