Where the hell to start with Deadpool? I think its best to say that if you loved the trailer and would be happy to watch another hour and a half of that trailer, you'll enjoy this film. The action sequences are exceptional and the cast (Ryan Reynolds in particular) have definitely spent time getting to know this particularly odd-ball super "hero", which comes through in both the humour and (constant) easter eggs. Personally, I was hoping for something a little more. Deadpool is anachronistic, immature and eternally comic but ultimately his humour and best writing are when he not just acknowledges his "meta" ness but fully grasps it. Fourth wall breaks are great, but the film was lacking that second beat provided for in the comics by the other voices in his head, that beat that questions why a superhero should behave in that manner or why the writer intended this outcome. That self-doubt and narcissism was present, but lacked the nuance that great writers can bring to the character, which was a shame.
And then there was Colossus. I get including the Russian metal-man, he is a big part of the Deadpool comics and a constant friend/foe of the titular character, but turning him into a walking caricature felt cheap. Yes, he is sometimes written that way in Deadpool comics, but that's because those comics are often written from Deadpool's perspective and to the mercenary Colossus is a simple, 2-D man of morality and brute strength, lacking complexity or intelligence. However, Petyr Rasputin is exceedingly complex and a very strong character in the X-Men's roster and, if the film was going for the "Deadpool-eye view" perspective it was not clear at any point.
Ultimately though, I enjoyed Deadpool. It wasn't everything I had hoped it to be, but it was much better than a Hollywood adaptation of ol'red'n'black'n'shiny has any right to be.