Belts are actually more common than one might expect even on large cnc routers. I've read that even under extreme loads on 8' tables they can maintain +-0.001" as long as you choose the right belts and tension them properly.
There are also alternative systems that combine fixed and moving belts for greater accuracy and speed such as this (
http://www.cnczone.com/forums/linear_ro ... er_if.html)
Belts are generally overlooked by most DIYers since many people assume that they stretch under load. If/when I build a cnc router I will be using belts because its much easier to ship them.
In terms of difficulty of mounting both solutions are similar, but if you want a belt with similar rigidity to a rack/pinion it will probably cost more (but shipping might make up the difference for long lengths). Lots of tradeoffs for particular situations.