We have stained concrete. We did it ourselves (disclaimer: My hubby does estimating for road construction, so he knows concrete inside and out). The work doing it yourself is tedious, but not difficult. We love it ! It's basically indestructible. The only issue we have is that it was worrisome when our kids were going through the climbing phase and when it's wet it can get extremely slippery. Other than that it's fabulous. We did our great room for a few hundred dollars.
We did look into hiring someone. The first poster is correct. A commercial company will want to put down a thin layer on concrete on your existing. This will make the coloration more uniform and covers cracks and imperfections. This is the main reason we opted to do it ourselves. We didn't want the extra cost or mess (no matter what you do, concrete pours involve a mess). We came up with a random pattern, so we weren't too worried about imperfections.
We also found a large crack in the concrete. We called out the engineers who designed our foundation. My husband wasn't worried, he was fairly certain it was normal shrinkage cracking. Since the floors would be exposed if we ever decided to sell the house, he wanted verification that the cracks were normal and not proof of a cracked foundation. So now we have an engineering report filed away. The crack goes mostly unnoticed by guests and most think "it adds character" when they do notice.
I am pretty sure I have a word document showing the steps we took to do our floors. pm me your email if you are interested and I will see if I can hunt them down.