I did a lot of research on this topic when we were remodeling our kitchen. I initially planned to make my own concrete countertops, and even went as far as building a table to pour on. Unfortunately, life got in the way, and we ended up going with granite. I don't regret it one bit, though - it got done very quickly, and the cost was very reasonable. I actually don't think I would pay someone else to make concrete countertops - there are considerably more variables to deal with, and I would not feel confident that it was done correctly, given that some potential problems might not show up for several years. However, looking at someone's work might convince you that they're competent, and if that's the case, then having them made might be a great choice for you.
With concrete countertops, you've got to be ok with some change in the appearance over time. They will stain and wear, regardless of how you treat them. That can be a good thing, if you appreciate the weathered look.
A couple of sites you should look at:
http://www.chengdesign.com/geocrete-countertops.htmlhttp://www.concreteexchange.com/