I used this system twice, just to prove to myself that it could be done. I don't have a suitably smooth piece of concrete at my disposal.
I have found that for pieces less than 6 feet long, I can do a pretty good job with a free hand cut. After a slab is cut, I run the saw along it, with the saw barely tilted, so that I am able to plane off any defects.
Thick slabs change dimension as they dry, so there is no point in going for a super smooth finish. Further working is required once the wood is dry.