This movie has everything you want and would expect, but it is surprisingly slow in spots. The story, the action, the comedy, and the special effects all deliver. But in this kind of movie, it's bad when you look at your watch. Fifteen minutes or so less would have made all the difference. I prefer the original, but it's a competent and enjoyable sequel.
The director really needed to decide what movie he was making - a dark comedy, a science fiction movie, a story of unrequited (if eternal) love. The plot, style and dialogue is a confused mix of all of these elements - which is a crime, as the cast is extremely strong. For a movie with so many likeable actors it was a shame not to leave the theater smiling.
The movie was a joy, especially Downey's performance. He really is the finest actor of his generation. You can't take your eyes off of him when he's on screen - and he's on screen almost all the time. Pretty much everything works - the acting, the effects, the screenplay. There is one scene in which the Downey character has his epiphany which rings false and you can detect the rewrite gang, but it's mercifully short. There's a franchise here if they want it.