My highest level character ever is going to make 12th level soon. I'm stoked; I can't wait. I've been playing D&D for 14 years.
The higher the player group, the more options the PCs can exercise. The more options the players have, the more fun the players have. When the players, and not the DM, can begin to control the flow of the campaign in terms of "What's our next move?" then you've got a great campaign. Players, uniformly, love being able to make their own decisions.
After 10th level, the DM's role starts to morph from coach to referee. You need to be more flexible, and you need to be able to adapt to what the PCs are doing. If you've ran the same campaign through to the current party level, then you should have plenty of established campaign information with which to interact when the players pull an unexpected side trip or manuever.
I think the intricacy, intrigue, and interaction of high level characters, monsters, spells, abilities, and magic items makes for fascinating gameplay, both on a mechanical and a roleplaying level. Yes, it's more work for the DM. No doubt about that. Aren't your players worth it? Mine are.