i have used shimano xt discs. i liked them a lot. they lever feel was great and the stopping power was ok, but i could not stop as fast as i can with the hayes i have on my bike now. shimano doesnt use dot fluid, i dont know if that is a plus or minus.
the hayes have a nice lever feel, not as nice as the xts though. the hayes scrap a little bit and make a slight 'organic' scraping sound. the hayes are really nice brakes, if you set them up good they can stop on a dime.
hayes for power, shimano for feel.