No one who might be offended by <insert just about anything> should see this movie. It certainly deserves its R rating.
Deadpool is all about the comedy, which makes sense given the character's comic-book background. Everything is joke, from the one-liners thrown around in battle to Wade trading insults at the bar. At times the humor is directed to the audience, "breaking the fourth wall", which I thought were some of the best bits. Even the romance between Vanessa and Wade was largely based on them tossing jokes back and forth. It works well in all cases, which is a tribute to both the writing and acting.
The plot is probably the weakest point of Deadpool. "Revenge against the guy who ruined your life, and oh look he's holding your girlfriend hostage" is about as unoriginal as it gets. I didn't really care, though, because the movie is about the characters, action, and humor. A complicated plot would actually have gotten in the way.
There's a whole lot of over-the-top violent action scenes in this movie. Not quite at the level of red-food-coloring-everywhere that you might see in a Tarantino film, but close. Deadpool's constant wisecracking makes these scenes comedic as much as they are violent action, though. I thought it was technically well executed. There are a few uses of slow-motion and such, but the special effects aren't over-used.
This film belongs to the X-Men film universe (as opposed to the MCU), and it does include a couple of other X-Men characters: Colossus and Negasonic Teenage Warhead (no, really, that's the name...it's not a Deadpool joke). But Deadpool is an origin story movie for Deadpool, and doesn't really cross over with the other X-Men films. Except to make a few jokes at their expense, of course. Also, Stan Lee as a strip club DJ was an inspired way to make use of the cameo he makes in every Marvel film.
Deadpool isn't a movie you can recommend to everyone. The viewer can't be too uptight about language, violence, or sex. But that viewer also doesn't have to know anything about superheroes or comic books to enjoy it, which I think is a major reason for the film's great box office success. I really enjoyed it, and hope they can pull off the same kind of performance for the sequel.