This might be the "grown up" Marvel. I'm guessing that the drunk guys who were in the theater on my night probably were the same kids who saw Spiderman back in its more innocent days. The ticket money numbers seem to suggest that the change in the franchise will be a profitable one. I guess you can't go wrong in the entertainment industry by being TOO cynical. It will be interesting to see what happens in the sequels. I'm guessing that the X Men will eventually ween Deadpool from his homicidal ways, and bring him out of the Dark Side.
Deadpool never becomes an X-Men in the comics. He does become part of XFORCE, think of it as an X-Powerd force that is willing to do all the things for the greater good that the normal X-men would not find ethical.
As far as being too cynical, that is his character, that is how he is portrayed in the comics. What I enjoyed about the movie is that it doesn't at all take itself too seriously. At over 280 million on a 58 million dollar budget, it will is profitable
My personal opinion, this should be a one off, I don't see the direction to go w/o a lot of the same, but hey, the same can be said from any Marvel Movie. Disney, or in this case, fox, is going to the well until the water is all gone and right now well is overflowing. Between Star Wars, DC, Marvel, don't expect a whole ton of originality in films. I am still mad that Disney cancelled Tron 3 however as I really liked the second one.