My sister is thinking about doing some DIY concrete countertops at her new house.  As an experiment, I wanted to try and make a coffee table with a cast concrete top to see how hard it was.  I built a mold out of melamine, then filled it with Quickcrete 5000 and steel reinforcing mesh.  After it had cured, I sanded the show side smooth and sealed it with an acrylic concrete sealer.  The base is red oak, and a simple (but sturdy) design.  Even that relatively small table top weighed a ton, and Lori and I had to work pretty hard to get it up the stairs.  All in all, I'd say it was a success.  I learned a few things that my sister should be able to leverage when doing her countertops, which will hopefully help her avoid a few (minor) problems I ran into.
