My house is over 30 years old. The fireplace was constucted from distressed white brick (just like the outside of the house). Numerous previous owners allowed children to eat on the raised hearth and the brick was badly stained. The front of the fireplace (from opening to ceiling) was just plain dirty from years of use/abuse. I had someone take a look at it to have it cleaned and the cost was going to be quite high. He did not gurantee a good results either. So, I went to a local, major home improvement store after doing a lot of research via the NET, and purchased a primer (I used the one that begins with a Z, name escapes me-Zinser maybe) and primed the entire outside of the fireplace (I did not even consider painting the inside out of fear of the paint burning)first. I used semi-gloss enamel (highest quality sold at the store.) I painted it in a light beige, covering all of the bricks and mortar, two coats. Then I painted just the tops of the bricks with a taupe color. It is absolutely beautiful. You can't even tell it is paint, it looks like natural brick. It has been two years, it looks great, basically like new. I have had many compliments and was asked by a contractor where I found brick that color (he was looking at it from a distance, granted). Anyway, that was my experience and I could not be happier that I did it. It is certainly better than looking at dirty, dingy brick.
Also, when I painted the tops of the bricks, I used several different types of methods for applying the paint until I found one that looked the most natural to me. I ended up using the sponge applicators and not applying the coat to the brick tops too heavily. It looks "distressed" that way, but clean!!!