The following review cannot contain spoilers because there is nothing to spoil in this movie, assuming you've seen the preview. It's a movie that proves the Marvel-movie machine knows how to make an adult-focused film that's still action, still superhero, and really do not take your kids to this.
It's ultra-violent and bloody. It's profane and borderline pornographic in places. It's rude and crude and assumes that you, the viewer, are OK with amoral, revenge-fueled violence and torture (typical Hollywood fare) accompanied by discussions of masturbation, anal and oral sex, sex work and stripping, nsulting blind people (who insult you right back), insulting peoples' weight and appearance, stereotyping (south Asian) Indian people, and cynicism at and from teenagers. The exchanges between Deadpool and Brianna Hildebrand's Negasonic Teenage Warhead alone are worth the price of admission.
What makes this good is that none of the above is stupid, and none of it insults the audience, though Deadpool does brilliantly break the fourth wall multiple times. Marvel even takes jabs at itself and its other characters/franchises. This film is not punching down - it's doing comedic MMA in the pit with everyone.
I also have to wave the fanboi flag for Morena Baccarin, who plays Wade's take-no-prisoners girlfriend. She gets too little screen time, in my opinion, and I'm hoping we'll see more of her in the sequel. I get that this is the origin movie and the key angst Deadpool has is whether he can ever return to her; this plot device keeps them apart for much of the movie and thus keeps her off-screen. In my opinion, what sells this thing is the extended scenes of the two of them together in the first half of the movie. I believe in Vanessa (her character) and I believe in Vanessa and Wade's weird/quirky/kinky relationship.
Finally, the use of Negasonic Teenage Warhead and Colossus in this movie give me great hope that the Marvel machine will take what it has learned from Avengers and use that to revitalize the X Men movies, which I feel have been flagging in terms of plot and how they're using the vast good material available to them.