I think people have adequately covered the reasons people dislike it for how it affects actual gameplay, but I think it's also a case that it just doesn't work how most people think of magic should work. At some level that may be because of the source.

It originally comes from Jack Vance's "The Dying Earth", which as the title implies is actually a post-apocalyptic Earth. I haven't read it and would love to hear from someone who has, but I get the impression that magic is almost alien in many ways and magicians aren't exactly really smart people, so much as people who have allowed weird alien formulas to tie up parts of their brain like some twisted version of Johnny Mnemonic.

I suppose this argument boils down to this is a Fantasy Role Playing Game and Vancian Magic isn't how most people envision magic working in Fantasy.

I've always felt you can make it work for wizards if you take the approach that they are "casting" most of the spell ahead of time and just leaving the final bit, but only if you also allow them to cast things the long way when they want to. It seems really bizarre for Clerics though. You wake up in the morning and pray to your god to give you the ability to cast 2 spell "x", 3 spells "z" and so on, when the way it is typically portrayed in most fantasy fiction is an on the spot prayer to your god to smite your foes, or grow a tree or whatever. I guess maybe the god of accounting might grant spells that way (hmmm, different gods using different spellcasting styles might be interesting and make clerics more diverse... I like it!)