Easy! (You don't need 8 characters to say that, iFixit!)
10 - 15 minutes
My computer had been showing the "service battery" warning for a while, and I was experiencing lots of strange power-related problems.
Replacing the battery was easy. Using the tools suggested, and following the step-by-step guide, it took me about 5 minutes.
Follow the guide. It is easy! Just pay attention to what you are doing, and nothing can go wrong.