Hay, I put in concrete counter tops in my kitchen and I love them. They look good and are cheap. I think I had to pay $60.00 in materials.
There are a few things to consider. You can pour the entire counter top in one pour if you want. I did cuz I am lazy and wanted to do the whole thing in one day. I formed mine with cheap 1/4" plywood cuz it is a material I am used to. You could use foam but make sure your seams are tight cuz concrete weighs a lot and likes to break thru things. I actually took blocking all the way to the floor to support my forms. I also used 3/8" rebar and spaced it about 16" on center. Maybe it was 12", I can't remember....but the more the merrier. Like others have mentioned, make sure your floor will hold up the weight. I have a steel beam under mine, but it is a new house so could allow for it. If you are in an existing home, you may have to beef up the supports under your floor. Talk to an engineer if you are unsure. I also used a warm tone color additive and some river rock which I wanted to expose and make the tops look sexy, but like a dummy, I used too much trowel and sent all the cement to the top and it got too hard to expose the aggregate. I mean I used a grinder until I about killed myself and I managed to expose very little of the rock. I also used a very rich mix of concrete. We mixed it in the mixer outside and brought it in with buckets.
Remember,concrete does not like to turn corners unless you provide a control joint. I used copper strips all the way thru the width of my counter and it has worked great. It breaks up the contertop into sections and allows for notch-outs for your cooktop and the like. I used them where the counter makes right angles, also. No cracks, except for some that form when it dries.....can't always help that. They are just surface cracks and are small and few and just add character to you counter.....but you may not get any.
I used a bridge sealer but you can use other water based sealants. I have found thru the years that heavy use areas, like where I have the chopping block, is hard on the counter top and I will have to reseal one of these days. I used a piece of foam to notch out for my sink. If you want to make a back spash, you will have to form them seperately and install them after the top. I used a tile back splash and it works great.
Now, if you want to precast the top, you forget all the above and make forms, cast your top in sections, and then place them on your conter top after they cure. It works good too. You have to be accurate so the different pieces set nicely to one another. You should consider a "key" detail that locks one slab to the other where the edges meet.
I just took a grinder to finish the edges. I wanted a rustic look, otherwise I would have used a more expensive material, like marble. In my case, with the mexican tile, the more rustic, the better.
A few months after I did my counter top, "Fine Home Building" magazine came out with an article with some really sexy counter tops using concrete.......you will have to do some investigating on the issue. It would be at least four years ago or so. Be worth the effort.
Good luck. Don't be too afraid to dive in. Also, remember to use "blue board" on the walls where things can get wet.