yeah, I've got a bucket of the Quikrete stuff and it works like a champ.
I drill and peg the rock with fiberglass rod (this last time I used a hand held grinder with a masonry wheel to shape the rock) or zip tie, once the structure is stable... thwap some hydraulic cement in the joints. You can shape it by pinching little crevasses or use a rock to crave it and leave imprints. I've read about some people using rock salt to add texture, but I believe that it makes the stuff too brittle. Only mix up what you can use in about 15 -20 mins the stuff sets up crazy fast. In the past I've a couple of small structures by pulling the rock out of the tank, join and gluing them and putting right back in, although if you are nervous soak the rock in some old tank water from a water change first.
PLEASE USE GLOVES! Concrete Burns are not fun.
Oh and do yourself a favor and don't rinse the bucket out inside at the kitchen sink, yeah the hot water is nice... but if that stuff sets up in your pipes you will never hear the end of it!