The 'wow, makes you think' indy ending *spoilers*
I watched this movie and almost allthe way through I was really enjoying the mix of dark humor, satirical realism, and absurd romanticism. Then they tacked on the crap ending. There are many ways it could have been tweaked to work (e.g., see my last paragraph) but the key is that the ending has to lead us to reexamining McConnel the man, rather than the reality of the world he lived in, because the entire film sets up a completely plausible world.
** Possibly Spoilers below **
The "all imagined" ending can be pulled off - it was the heart of "Vanilla Sky" and has been well done in many others including "hero with psychological problems" films from Donnie Darko to Jacob's Ladder. The typical path to pulling it off is plot progression riddled with implausibilities and discontinuities setting up the question of "what is real and what isn't". To pull it off usually requires the injection of some science fiction or fantasy elements into the story, so that you can beleive the "way out" stuff just might be real.
HWAQM fails miserably with its ending because even the "way far out" stuff like talking fish (really the only thing) is in fact possible under the premise of the film (talking fish = product of psychosis). Perhaps the people involved failed to realize how true-to-reality their fantasy world of corporate cogs and football tossing frat-boys actaully was. There are no talking animals in Vanilla Sky, but you know something is wrong when the sky is always crystal blue, when there are absolutely no cars in NYC, etc. Even going all the way back to The Wizard of Oz, there is one fantasy construct after another in the dream world of Oz, all of which are confined to the Oz universe, but could never happen in the real world of Kansas. There is nothing like that in HWAQM.
When they throw one of these endings on a film that could have been entirely plausible in the ante-fantasy reality, it makes you want your 90 minutes back. Take Repo-men: they do have some over-the-top scenes, including the final scanner scene, and then they try to pull back the curtain and say, "did you really think that could happen?" Well, actually, yes, we did because it was plausible under the rules of the universe set up in the "pre-dream" part of the film. Weak use of a familiar ending scenario.
The only film I can think of that comes close to pulling off the "shocker reveal" with a plausible guy going crazy at work under completely realistic mental distress and no science-fiction plot elements might be The Machinist.
In the same way, HWAQM sets up entirely plausible scenarios laced with a mixture of bitterness and hope. So he talks to his fish, but he knows the fish can't see anything he doesn't see. He is incredibly unhappy; well, it is an unhappy world. If they wanted to unravel McConnell at the end, there was no need to undermine the rest of the story with an "it never happened" ending, other than to be typically "indy" and "blow your mind". Let him go back and have the confrontation with the CEO. Show Vanessa WALKING by intact - introduce the possibility of a genuine element of "dream" discontinuity. Then show him in the parking lot, but there is no Lexus, just his old Toyota. Send him back to where he thinks Vanessa's house is - but it's not there, maybe something dramatic like a giant hole in the ground. Then when he walks in to the cubicle farm again, it is clear he has gone back in time to the original shooting. He could start asking frantically for Vanessa. People he asks start to look disturbed; someone goes for the company shrink. He pulls out his gun. Run through the same "suicide" sequence, then have a final cut away to a picture on his desk of Vanessa in a wheel chair, belt holding up her forehead and all. Now the film does "make us think".