I've seen only two episodes (from the first season). Quite engaging and spectacular. Fairly good entertaining. First episode (probably the pilot) more than the second. However, the second had this wonderful graffiti in it. (Broadcasted on BBC mind you!) So, obviously, I liked it more ;)

The characters talk a little too fast for me (not a native English speaker) and the whole tempo of the movie is a little too quick, which of course makes it a more intense experience, but also makes the viewer feel a little confused (like an "idiot", to cite the new reincarnation of Sherlock Holmes) but also worry about the plot's imperfections, abrupt "gaps" and "jumps". To give just an example: Sherlock notices an inscription on a watch, that he sees for a few seconds, but he doesn't notice for hours a a couple of crucial handwritten words on a big piece of paper on the table in front of him.
I don't remember anymore the original story, but I always though that the original Sherlock was the kind of guy concerned above all with people' lives. He would always hurry and try to save them from the killer. Here, Sherlock is more about fighting with the enemy and less about preventing further homicides.
Also, Dr. Watson killed a person in both episodes. Saving lives both times, nevertheless this makes him a serial killer too.