Personally, I prefer displacement maps to bump and if you look at the both the bump and displacement sockets, there is a settable amount for them. For example, using Brycetech's bump/trans maps (from the Bryce skin tutorial) on the displacement channel requires a setting of about .03 or the skin goes all craggy. So the moral is, turn it down until it works right. Keep in mind that displacement maps displace the actual geometry and will look a little more photorealistic in raytrace renders, at the expense of some more render time.