It's difficult to answer that question seeing as the series has so many games.
Others have already said that it is, in fact, not historically accurate and that's correct for the most part.
The historical periods, recreation of cities and some of the more vague and general information are often pretty close to reality. For example, I loved playing Assassin's Creed 2 and Brotherhood and pointing at the screen every minute, saying "I've been here, I remember this church/square!" When I played Revelations, I was attending a class where some of the topics were about Constantinople and I had a pretty good time, being able to recognize the city plan, where everything is and the names of buildings. Because I've seen them in the game.
However, other than that superficial accuracy (architecture is easy to recreate; we know where stuff was mostly because it's still there), the events and people fall into three categories: 1. Existed, but changed completely; 2. Loosely based on something that existed; 3. Didn't exist
A lot of events that happen in Assassin's Creed are based on something real, but are thoroughly changed to fit the fantastical narrative of the games. The games also used some general events, places, objects, myths, legends and individuals, and gave them totally different roles that fit the game's narrative. Someone up there mentioned Da Vinci's tank for example; Da Vinci truly designed something like that, but of course, nothing of the sort has been made (in the past at least).
Basically, you'd have to inspect every detail of every game on its own to see how accurate it is. In most cases, you won't be able to learn actual history from these games, but they might spark an interest and prompt further reading about a historical period or a historical individual. I'm an archaeologist (not exactly the same as a historian, but still) and I always found Assassin's Creed appealing because it deals with history, no matter how changed. I've heard from many people that they became interested in history after playing the games and that's great. However, I definitely recommend not taking anything history-related from the games at face value. The series is entertainment primarily and takes many liberties when it comes to portrayal of historical periods, events and people.