Interesting Experiment

Rating: 6/10 (9 March 2015)

Worth seeing for movie fans, but mainly for the technique, not the content.
Inarritu pulls out all the stops, shooting in almost one seamless take. (Spotting the cuts is about half the fun - unfortunately.) Cinematically, the film is arresting, almost breathtaking. But only barely entertaining.
The acting is superb, throughout. Micheal Keaton is excellent as usual, though I'm not sure this is his best performance ever. (He was at least as good as the charismatic psycho in Desperate Measures, for just one example.) It's a shame he was passed over for the Oscar, he's one of the best actors in Hollywood, and probably overlooked mainly because he's associated with comedy rather than drama (though he's done both superbly).
The problem, as others have pointed out, is that Birdman just doesn't add up to much. (MILD spoilers, nothing to agonize over, especially in this largely 'plotless' film...)
Keaton is frantic, insecure, vainly trying to take his career where it probably doesn't deserve to go. But you get that in the first five minutes. After that, you have to grab your pleasures where you can. For example, the underwear scene is fun, but it works as little more than an aside. Its strong reception from the Broadway audience doesn't seem to greatly affect Keaton's approach to his play. The sub-plot concerning Keaton's daughter is similarly wasted, as is the beautifully ambiguous performance by Ed Norton as Keaton's bombastic co-star - a character who totally dominates the first half of the film only to go strangely missing in the last act.
The pay-off for all this is minimal. What does the ending mean? Bottom line, not much. All the technique lavished on this threadbare story can't replace the substance that's just not there.
Birdman reminded me of Black Swan, another bit of overblown Oscar-bait that offered great technique and lots of angst in lieu of saying anything particularly novel or interesting about the theatrical life. Birdman is a much better film, and a far more entertaining one, but still not exactly a must-see.