I watched 27 minutes of The Plainsman (1936) and found it unpromising. Then I looked up Cecil DeMille and was a little surprised that I had never seen any of his work, other than his cameo in Sunset Blvd.
The Big Clock (1948) starts slow but pays off quite well.
I enjoyed the first half of Jekyll (2007) more than the second half.
Steven Moffat has a habit of building up to disappointment.