In my experience with AMD Crossfire, it is very dependant on drivers which contain the Crossfire scaling profiles within. When it does work, the results are usually great, but this is few and far between. You will basically have to resort to using one GPU only if there is no crossfire profile, or the one that exists is not very well optimised for a particular game or engine. This happens more often than not.
In short, I would not use crossfire again unless the second GPU was a really good price that I could not pass up. I would focus more on getting a good single GPU.
Crossfire does obviously require a larger PSU to run two cards, so it is worth thinking about that also.