One should note that there are many different brands of 2-part epoxies in the plumbing section of the hardware store. Many of these are toxic, and many set MUCH faster than kneadatite. Not to mention I find the consistency of other epoxies harder to work with. I\'d recommend staying away from most of these unless you use proper protection.
Kneadatite is non-toxic, sets somewhere between 30-60 minutes (depending on temperature and mixture), is an easy-to-work-with consistency, and for these reasons is the medium of choice for most sculptors. The commonly used forms are blue/yellow (green stuff), white/blue (white stuff that GW sells repackaged), and brown/gray (brown). Green is moderately flexible when cured, and is good for general use. White is a little harder and a little easier to file to a hard edge. Brown cures very hard and is great for filing, drilling, or whatever. Of course, the harder they are, the more expensive they are, which is another reason people use green stuff more than anything else.
Putties can also be mixed together for inbetween hardnesses.