I've read about half of Darwin's Origin of Species. Out of sheer curiousty, and because I heard he writes really well. It was very interesting.
For scientific purposes, a modern biology textbook would be better suited. Evolutionary thinking has progressed a lot since the days of Darwin. (One doesn't read Newton for the last word in physics either)

I've read most of the bible too. Didn't finish the new testament. Some guy in church spoiled it for me - he revealed the hero dies in the end.