The first 2/3 of this book offers up a remarkably rich and engaging narrative combined with a nice mix of historical/literary detail. I'm afraid Simmons loses this nice balance in the final act, however. The story gets deeply bogged down with odd (and seemingly irrelevant?) historical trivia, losing much of its earlier interest and impact - making for an uneven experience overall.
Ultimately, I'd still recommend this for fans of the era, the literature and most of all fans of Wilkie Collins (and yes, of course, Charles Dickens too). However, I'll caution you that if you are an ardent Collins fan (like myself) you may find yourself (somewhat unreasonably) chafing a bit at how negatively Collins is portrayed here. It is my opinion that his writing has withstood the test of time far better than Dickens' has.
Enjoyment will be greatly enhanced if you have read or are familiar with some of Collins' and Dickens' work - particularly The Moonstone, The Woman in White, and Our Mutual Friend. I'm not much for reading unfinished books so I cheated and rented the most recent BBC adaptation of The Mystery of Edwin Drood just so I could familiarize myself with the general plot and characters.