I'm one of the few on this forum who actually has concrete countertops. You'll hear a lot of "my neighbor/friend/whatever has concrete countertops and...." but there's not a lot of first hand experience with them.

Below is a link to a previous post of mine on the topic. I don't have much to add to it, but will be glad to answer any questions you might have. I opted to go with undermount stainless steel sinks so I can't really comment on the durability of the cast concrete sinks. They look beautiful and do allow you to easily incorporate features like runnels. I was just concerned about how the sealer would hold up to the abuse that a sink is subject to. And I also wondered what would happen if I dropped a heavy cast iron pot in a concrete sink. For that reason, I opted to go the stainless steel route. Having already dropped a very heavy soapy pot (leaving an ever so slight dent in my stainless steel sink), I think it was the right decision for me.

Here is a link that might be useful: Link to earlier thread

We have many customers of ours who make concrete countertops. They are nice and with regular sealing should hold up well as long as they are laid right and all air bubbles are removed. I would think about using an undermount sink instead of concrete for the sink just so you have more durability there.

I just poured my own last Wednesday, and am a little nervous about what I'll find when I take them out of the forms. I poured them upside down with melamine forms, so I didn't have to trowel all day.
I've heard sealing stories that range from every couple of months, to every couple of weeks. I'm not sure what to think; hoping that polishing them from the get go will help.

I used to have concrete kitchen counters that were installed in 2003. I still have concrete counter in the master bathroom. The colors are amazing. I was unhappy with my chocolate brown kitchen counter. It was fine for about 15 months and then it started bubbles of light cocoa brown which erupted wherever lemon juice or acid foods came in contact. Installer had sealed with carnuba wax. It was never right again. When I had the slab leak last summer that destroyed the lower cabinets, I opted for soapstone counters. In the bathroom, my green and apricot concrete counters remain beautiful. From my limited experience, I would not use concrete again in a working kitchen.

What leaked? Was there a seam?

Thanks so much for the responses, and nclakehouse's link was very helpful. Sounds like I should forget about the "plus sinks" part. Cabinfo, it would be great to see pictures when the countertop comes out of the form.