Games to film are notoriously utterly diabolical, it is usually better to disembowel yourself with the nearest blunt object rather than watch a film made from a game. This is the contradiction that proves the rule correct.
My main worry was that this was going to CGId to heck. CGI being the worst thing that has every happened to film. Dodgy superimposure of earlier films usualy gives a better result than most films "attempts" at CGI. Assassin's Creed however has used the CGI in an understated manner. It is there but is not offenisve to your viewing pleasure.
So, two ticks there...but...
There are some elemenents that seem to drag on in the film, beyond what is necessary. Two scenes I thought "ok, this has been going on long enough - lets move on" but for the most part the story runs nicely and smoothly.
Last thing, the credits are 15 minutes long if you are looking at runtime for the film.