I have had an absolutely terrible experience with it, which sadly isn't unique amongst these big PHP projects.
My primary job for the last 2 years has been developing a Drupal site for a group of daily newspapers with monthly page views in the low 8 figures.
We were in a situation where we had to get a site up quickly (Our small group was bought off from a much larger conglomerate, so the huge $$$$ Java system we had been using
went away). Drupal allows us to get a tolerable site up in about 3 months, and a much better site up about 6 months later. Have there been pain points? Sure. But if we'd used anything else there's no way we would have been on our feet nearly as quickly.
Are we investigating other options? Of course, if I wasn't I wouldn't be doing my job. There's a part of me that would love to rewrite the whole thing in Rails or Django. That would be a huge undertaking though.
We've had different experiences, but I wouldn't touch it with a 10' bargepole, not any more. And I'm glad I've been managing to encourage my boss to start moving away from it. The only positive thing I've been able to take from it is a list of things never to do in my own code.
Like I say, it's not just Drupal. It's a side-effect of over-complication in the name of simplicity, and trying to run in parallel a system that makes it easy for non-devs to use. It's a recipe for disaster if you want lean, maintainable code.
Oh, and it causes performance problems too.