If you weren't very familiar with the character of Deadpool/Wade Wilson before (which I wasn't), then surely you've at least got the gist through the film's ubiquitous and hilarious marketing campaign. Deadpool is a disrespectful, R-rated, jokester badass whose wit is even sharper than his bloodlust. It makes for a movie that's welcomely different from most superhero fare.
Most of the time. For the most part, Deadpool is a film where the jokes come a mile a minute, usually pausing only for a bit of over-the-top violence. When it's doing that, the movie is wildly entertaining. It's at its very best in the moments where Deadpool breaks the fourth wall to directly address the audience, or make a meta-reference that belittles some sacred cow of the Marvel universe. Even the opening credits are a prolonged, perfectly executed joke. The humor in this movie totally works, and it's hilarious.
But there are a couple sections where the movie falls back too earnestly on the very conventions it seeks to lampoon. Deadpool's origin story is at times (as so many superhero origin stories are) a bit of a drag on the movie. The romance aspect mostly works, as Wade Wilson is paired with Vanessa, a character as sarcastic and fun as he is. But the actual "making of Deadpool" flashback in the middle of the film is an offputting smorgasbord of torture sequences that feels at least twice as long as the 10 minutes it probably is. It's not giving the finger to convention, it's revering it. And reverence does not look good on Deadpool.
Fortunately, the movie is always able to regain its momentum after these few scattered missteps. That's thanks in large part to a perfect pairing of actor and character in Ryan Reynolds. He's played both lovable and smug before in separate movies that never seem to make the best of either of those qualities; here they finally come together at the intersection of a perfect Venn diagram. Morena Baccarin (beloved by Firefly fans) is an excellent romantic foil who is more than a match for him. Other highlights in the cast include T.J. Miller (basically doing a version of the character he always does, but he's good at it) and Ed Skrein (who is basically a stock comic villain, but an effectively menacing one).
Overall, I'd give Deadpool a solid B+. Depending on how much you've been caught up in the hype, it might not quite live up to your expectations. But you certainly won't be disappointed.