I only ever actually ran a game with XP once and that experience (heh) was enough to decide to never do it again.
For one, every session had to end early for me to tabulate all the XP. Oh, I started out with an XP budget for everything I had planned, but then the players did other things, ran into different encounters than I planned, accomplished things I didn't expect and so that plan when out the window. So instead we just sat around the table waiting for me to do math and guessing instead of doing something fun like playing more or making a run to the seven eleven at 1am.
And the fact that my group was creative and unpredictable meant the XP they then earned made them level unpredictably. They would bypass a chunk of something and then not be ready for what came next or they would be super extra and then rofl-stomp everything. Keep in mind this was before I discovered that CR is basically a Secret Test of DM character where you pass by learning not to use it.
Then they started to cotton on to the fact that sometimes their cleverness screwed them out of XP (because based on how the 3e DMG put it, they could get XP for finding another way to solve an encounter gave XP, but if they never run into that encounter because they went around Robin Hood's barn to get to the thing it was leading to? Nope.) and started arguing among each other to be less clever and act less in character so they wouldn't die.
This and seeing how XP was used in other campaigns gave me a nasty 'operant conditioning' taste in my mouth where it felt like I was expected to train my players with XP like a scientist trains a dog with treats and... these people are my friends; I don't want to feel that way about them.
Finally, I was rereading the DMG and realized that you don't level as soon as you ding, you level when the DM says you can spend XP to level. So I was like 'why am I doing all this work when I'm supposed to say when to level in the first place?'.
So I stopped.