They are, by and large, evil. That's why Stroustrup discourages the use of macros in particular and the preprocessor in general. That's why practically all newer languages got rid of the preprocessor (while copying many other concepts and syntactic stuff from C/C++).

I'm sure you can find plenty of literature on the subject if you are really interested. The points made by Mark Wilcox are valid. Just because people manage to cope with #ifdefs does not mean they are the best (or a good) solution for the problem. One of the greatest achievements of Qt was and is that it got rid of a hell of a lot of #ifdefs, increasing productivity (and testability) along the way.