I painted our old fireplace (with "Bricks Anew" kit sold on line) that had seriously ugly black mortor and old ugly bricks. It took me several months to get up the nerve to actually paint the fireplace as I had the same concerns many people do...once you start there's no turning back.
I finally did it and can't say enough about how happy I am with the result. I have recieved many compliments on it and it has completely transformed the room. The kit is very very easy to use and it goes pretty fast once you get started. The thing is that you control how much you want to put on...I started with the mortar and base coat over the brick. I left a very slight amount of the brick color showing so it didn't appear too solid. Step back every once in a while and see how you like it. After that you apply the other colors, again apply a smaller amount, step back and apply more if you like or change to the next color. There is a very slight difference in the colors they give you (atleast for the kit I chose) so you can't make a terrible mistake. I say...just do it! You WILL be glad you did.