I've made a bunch of rock using a recipe similar to what's suggested on the garf page. If you get creative you can make some pretty cool shapes, but it does need to cure for at least a month and ends up being a lot of work.
That said I thought it was fun to make and a cool spring/summer project. If you do try it remember to use some heavy rubber gloves or your hands will be peeling for days (and it doesn't feel very good).
I got the best results mixing in a good porportion of crushed oyster shell and keeping the mix on the dry side so that it's clumpy instead of soupy. The rocks tend to break up somewhat easily if handled roughly but once situated in the tank they don't fall apart.