This movie did a great job with telling the story that prequelled the World of Warcraft MMORPG. With that in mind, this movie kind of assumes that their audience is already familiar with at least SOME Warcraft lore. Whether that's from WoW or the original RTS games. As such, if you're not familiar with any Warcraft lore, you might find yourself a bit lost on some of the details they mention. In which case, you might rate this movie more like a 4/5 or a 3/5.
However, if you DO know some lore, then those mentions can be quite enlightening and enjoyable. I don't know if this movie is canon exactly. It does talk about several pivotal moments that ARE official canon within the games, but I doubt that all of it was canon. Even if that's true, it was a very enjoyable movie with great CGI. Though it focused mostly on the Alliance side of things, the Horde also got their fair share of screen time.
Considering that they've made nearly three times the money that they put into the movie (~450 million at the time of writing this). I'd say it's safe to assume that the movie is much better than what Rotten Tomatoes and other critics have said about the movie.
I'd say it's definitely worth a watch. I'd also say that I can't wait for a sequel!