I guess it's reasonably useful for mockups, but you can not seriously design a website (or any UI) with absolute sizes.
There are plenty of advantages to relative sizing, and in general it's a good policy, but there are many excellently-designed web sites whose sizes are absolute – just as there are many shit designs that're responsive as hell.
I started with basic min/max widths, because that was the sizing I commonly used. Percentages are important also, which will be coming.
Plus our experimental logic/animation tool let's you do some pretty spiffy stuff: http://www.screenr.com/Z9o and should support media-query conditionals in the future!