Okay, first, one of the problems with reading this book in this day and age is that this book is mainly a mystery that hinges on the fact that you, the reader, don't know that Dr. Jekyll and Mr. Hyde are the same person. Well, of course, in 2011 we all know the basics of the story. Reading this book is almost a pointless errand because the big final reveal, the solution to the whole mystery, is in the final chapter where Stevenson finally tells you that Dr. Jekyll created a potion that turns him into Mr Hyde, an alter-ego through which he can act out his baser urges.
They never go into detail on these baser urges but I'm guessing whores. And Opium. And beating up kids.
On the plus side, this book is barely 100 pages in the volume I have. You can read it in an afternoon. It moves fairly quickly and there's lots of great creepy details and atmosphere.
The afterword in the version I own* has some cool thoughts about "Who is Mr. Hyde?? WHO IS HE REALLY??" and it psychoanalyzes Robert Louis Stevenson. I love that kind of stuff. It also points out a couple questions from the story, stuff that doesn't have a hard definitive answer. At one point, Hyde kills an old gentleman but we never find out why. We don't have that piece of the puzzle. It's left up to the imagination. The book has a couple cool parts like that.
So I'd recommend it but only if you're a victorian or horror nerd. You know, like a steampunk kid. Or an Alan Moore fan. Everyone else: STAY AWAY. This is not for you!