I prefer the Aquascape epoxy (blue box, has a greenish putty and a pink putty that you mix). I'll roll a ball of it loosely together and then stick it in the microwave for 10 seconds to soften it up, and then work it together for a few minutes to completely blend it.
The best way to secure the rock is to drain a good amount of the water out of the tank into large (clean) garbage bins, so your rockwork is exposed (you can aquascape it this way, its usually easier to see) and then put the epoxy in cracks near joins in the rock. Stuff it in and press to form it against the rock. This tends to work better than squishing it between two rocks. I'll use quarter-sized balls all over the rock in as many places as I can and then fill the tank back up with water. It will cure underwater and I'd give it at least a day to fully cure before testing it
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Its never 100% though, if you ream on it or press too hard even after its cured it could break apart. But I guess its a trade off, its nice to be able to move the rock incase you need to.
-Diana