@Moose They way I feel about X3 is about the only way I can imagine how the haters feel about man of steel. X3 was an affront to the franchise. Apart from a few tiny good things, they insulted a lot of core characters with it as well as complete stories.
The big difference between a reboot and a sequel is that for a sequel expectations ARE validated, because a good sequel respects it's predecessors and builds upon what they made in a way that's rewarding. A reboot can do whatever the **** it wants and you either like a new direction or you don't, there's nothing a reboot has to live up to critically speaking (not withstanding many would still like it to of course), a reboot doesn't 'owe' anyone anything as it is at its core meant to create a fanbase, not serve an existing one (although again, we almost always hope a reboot serves the source material, but opinions can obviously differ on whether it does).
X3 is to this day one of the most disappointing experiences in cinema I've ever had, rubbish, rubbish film that used both its characters and its story in a horrible shallow classic hollywood BS way.
Also, X3 already teased that Xavier was alive and that Magneto got his powers back, so what do you keep? A seriously botched and lesser version of a dark phoenix saga + a wolverine that's not wolverine anymore + a shafted cyclops. Singer kept everything new that was okay in X3: Kitty, Beast and Wolverine's killing Jean out of necessity.
I'm glad they found such a classy way to retcon the franchise, as I geeked out of my mind when Wolvie woke up at the end. So rewarding. Reminded me of Inception a bit and I feel this film on a meta level came a bit full circle as Nolan made his Batman because of X1, and I'm pretty sure Singer's been inspired by Nolan in turn.