My tale is not so much of an experience as it is of a particular player. He's an experienced RPer and absolutely loves to write rich histories and backstories for his characters. When it comes to his style of play, however, he's far more of a powergamer.
My first experience actually playing with him was a 3.5 campaign run by a friend and themed directly upon The Elder Scrolls: Skyrim with the base rules tailored to suit that universe, down to available races and classes. The aforementioned player dug deep into 3.5's library of supplements and found the expertly-worded and ever-confusing Half-Dragon template and somehow convinced the DM to allow him to use it. Neither I nor the DM understood how it was intended to function; there was much confusion regarding the hit dice that were meant to be used, what "monster levels" meant in the leveling scheme, etc. The player took advantage of our ignorance and interpreted every last word of the template to be in his favor; the resulting monstrosity of a player character had about triple our hit points and strength beyond belief (I think at the end of the day his bonus was ~+8). In the week between character creation and campaign start, I conferred with the DM over my concerns, having spent hours reading and re-reading the cryptic template entry. I discerned that, yes, he should have crazy bonus strength, but the monster levels should have detracted from the class, not stacked with them, resulting in an archetypal "glass cannon." As the character was created and we were on the verge of starting, the DM just allowed it for the sake of keeping everyone happy.
By the third session of this short-lived campaign, the half-dragon had successfully gotten the kill shot on nearly every monster, forcibly opened the gates of two major outposts, and somehow forged upwards of 100 swords in a week's time (The DM also didn't understand crafting), making a ridiculous profit which was subsequently spent on crazy overleveled gear.
But that's not the end, oh no: After that campaign crashed and burned, we started to get the tabletop itch again, so I volunteered to DM my first campaign, this time in 4.0. After reading every book I could find, I found the system much harder to break in the manner the half-dragon attempted to with 3.5. For an extra layer of security, I implemented a point-buy system for starting abilities (this player was also fond of "cradle-deathing" characters with unfavorable ability rolls) so everyone would be on a much more level playing field. His choice of character? A Dragonborn Warlord with a STR of 18 before his race's +2, but naturally. Everything was kosher, though, so after I helped my wife with her powers and the mechanics of this new system, we dove into some first-level module that I forget the name of. He played his character well and everyone was having a good time hitting things with sharp sticks until we encountered our first "boss" enemy, a massive orc warrioress. I wasn't being too picky about targets, frequently selecting them either at random or based on damage dealt. The orc approached our Warlord and used what I believe was her encounter power, some crazy two-weapon attack with a pile of side effects.
I rolled to hit.
I critted. His Dragonborn was irrevocably splattered against the wall.
My first PC kill. And it was the one player who most thoroughly enjoyed having overpowered characters.
On the surface, I did feel a genuine sadness and apologized profusely as one might for running over a neighbor's cat. It was my first kill of my first campaign, after all. But deep down inside, and once we had retired for the night, I was giddy as a small child receiving an NES on his birthday.
I let him roll up a new character for the next session. His choice? A Dragonborn Warlord, this time with "only" 18 STR and a bit more CON.