Deadpool has long held a special place in the Marvel comics universe - an obnoxious, motor-mouthed anti-hero who frequently breaks the fourth wall by talking out of the comic to the audience. Ryan Reynolds played the part both in the appalling "X-Men Origins: Wolverine" and in this movie - although his origin story and final appearance are significantly different. I think Marvel would prefer we forget about "X-Men Origins: Wolverine," even though Hugh Jackman is still playing the role of Wolverine. So let's consider this an entirely separate movie.
Reynolds plays Wade Williams, a former special forces operative and current mercenary in New York City. While he talks a lot and is unpleasant to be around, he's shown doing good things for little or no money. He finds a soulmate in the equally obnoxious prostitute Vanessa Carlysle (Morena Baccarin), but a year into their romance he's diagnosed with terminal cancer. This eventually leads to his entering the "Weapon X" program (was it ever actually named in the movie? I don't think it was) where he's cured and given superpowers ... but the price is very high. Which sets him on a path of vengeance.
The movie starts around the mid-point of my plot summary with amusing credits. (It eventually backs up to fill in the details, all narrated by Deadpool.) It then proceeds into a fight on a freeway that had me laughing so hard I actually paused the movie because I was going to miss the next thirty seconds of the film. Reynolds, who desperately wanted to play Deadpool in a movie of his own, has been handed a great script and is having the time of his life delivering the funniest material ever given to a guy in a superhero costume.
My biggest single problem with the film was T.J. Miller as Weasel: he's supposed to be Wade's best friend, but all he ever does is drink with him and insult him (I understand that this may be their dynamic - but still). This is supposed to be the "comedy relief" (it says so on the case), but I didn't find him very funny and he sure as hell wasn't a friend. On the other hand, Baccarin was an inspired choice as Wade's love interest: she's beautiful, but also sells her foul-mouthed behaviour and attachment to Wade. The fights are good not just as "superhero" fights, but also as platforms for Deadpool's jokes, and the people who scripted the fights clearly revelled in the opportunity to do things that they couldn't put in other more mainstream Marvel movies (Angel's brutal and hilarious crotch shot on Colossus comes to mind, but is far from the only thing).
Has a very distinctive style and is perhaps not for everyone, but most fans of superhero movies will enjoy this incredibly irreverent and hilarious ride.