I've played in two campaigns that went into epic territory and am DMing one that is around level 15.

The two games I played were high OP and the one I'm DMing is that players can do anything RAW.

In the campaigns I've played usually one player (usually a caster) ends or mostly ends the encounter in 1 or 2 rounds. Occasionally there is a huge rules discussion on some obscure weird interaction. Stock monsters and tactics don't do too much to the party but woe to the new guy who shows up with a single class Tome of Battle guy. They arnt going to have a good time. Monsters disable or debilitate players pretty fast and tactics often involve surprising enemies. At high levels the whole "get a quest do a job" starts to fall apart because characters can use metafaculty, greater teleport etc. And they just teleport into areas. Disjunction gets thrown around a lot which slows the game down because all attacks and macros need to be changed to take into account no magic. Players become immune to most if not all status effects. Antimagic fields become used a lot more often.

Usually DMs end games about this time because it's too hard to prepare for games. Building custom monsters with class levels takes too long, characters can at will ignore prepared material or trivially solve most problems.

Players have a lot of fun. They are able to do all sorts of silly stuff and able to challenge demon Lord's, gods, powers that be. They arnt constrained to a city or town. They can decide they want to find a god, recover an artifact, and kill all the dragon in x,y,z area. They can become kings, princes, or whatever.