There's no doubting that Aronofsky had a vision in making this massively ambitious film, and had it come off, The Fountain would have been dizzyingly great (he's certainly got it in him - as discomfiting as it at times is, Requiem for a Dream remains a flat-out masterpiece and Pi, while flawed, isn't all that far behind), which makes it all the more a pity that instead it's a bloated, tedious mess, albeit one in which the glimmerings of a great film are clearly apparent. The failure certainly doesn't rest with the actors - Hugh Jackman is mesmerising, and Rachel Weisz, who has less to work with, pulls her weight too - and nor is it to be found in any particular element of the wider composition of the film...perhaps, the conceit is too large to be effectively rendered in an hour and a half of cinema (though it must be admitted that there's a pleasing ambiguity as to exactly what - and whose - the conceit is, and that some interpretations work much better than others).
Incidentally, I also found The Fountain a bit gruelling for reasons entirely unrelated to those recounted above, and considered stopping midway more than once on account of these other reasons...cryptic, cryptic; after a hiatus of a few months, I think that it may be time for me to begin keeping my (handwritten) diary again.