So I just came back from watching Warcraft, and I have to say, it was freaking awesome!
I can understand why the critics didn't like it: most of them probably couldn't tell warcraft from skyrim. The movie had a frenetic pace, and I can see how it would be easy to lose the plot if you weren't already familiar with the story. A lot of plot elements in the movie weren't really explained, such as why Medivh was such a douche, or why there's an old lady in a cube. However, to those who know the lore, it all makes perfect sense.
After watching this movie, I have gained a huge amount of respect for Blizzard and Duncan Jones. They could have taken the safe, easy way, and told a classic story of good guys overcoming adversity and defeating the bad guys in the end (and in the process probably mangle the Warcraft lore beyond recognition), but they didn't. Instead, they stuck to the story of Warcraft 1, even at the risk of alienating the general audience.
So if you're a Warcraft fan, go see this movie. Do not hesitate!