My most recent campaign ran to 14th level before I moved away, but I felt it was getting more enjoyable as the PCs gained power. As DM, I felt that higher levels gave me more freedom to throw challenges at the party. The PCs were gaining reputation and becoming significant decision makers, taking part in councils between ambassadors, meeting with rulers of the kingdoms, etc. I found that the scope of adventure possibilities was broadening, giving me greater flexibility and more creative license, and the impact of those adventures was increasing, giving the players greater satisfaction in the accomplishments of their characters.
Combat did become more complex, but it was still tactically interesting and not overwhelming. For example, our group had 7 PCs, an NPC, an animal companion, and a cohort, for a total of 10 party members. A battle between the party and 3 grafted Frost Giants, 4 Winter Wolves, a Frost Sorceress, an Elite Advanced Winter Wolf, and 2 Frost Giant Zombies that lasted about 8 rounds took us 2 hours to play out. I find that to be acceptable, as it was a big, significant encounter and still left us with 2+ hours for the rest of the evening.
I found that dungeon settings still had their place, too. Although the party could teleport, they couldn't do so with just one spell, and the chance of a mishap made it risky. In fact, the physical distance limited Teleport as well, as they had traveled over 1500 miles from where they wanted to be.
Prep time had increased, but not to an unreasonable level. For the adventure that featured the battle above, I had the following:
Grafted Frost Giants - I had three varieties of Frost Giants, each with different grafts and different ability arrays. It took about 10-15 minutes to stat up each variety, for a total of about 40 minutes
Winter Wolves - Standard from MM, no prep time
Advanced Winter Wolf - 10 minutes
Frost Giant Zombies - 10 minutes
Frost Giant "Jailer" - 30 minutes
Human Sorceress - 45 minutes
Ice Devil - Standard from MM
Some corrupted creature - 10 minutes to apply the template to the Fiend Folio stats
So total prep time for the opponents was about 2 1/2 hours. That covered 3+ weeks of gaming. I had started plotting and planning for the adventure weeks before the party got there, so it wasn't much of a burden at all.
I have created a "library" of high-level NPCs for use as needed, and there are a lot of online resources (such as
this one) to provide NPCs, so it's not too much effort to create NPCs when I want them.
Overall, I find that my enjoyment has increased with higher levels, rather than decreased. There may be a point where this starts to reverse (probably around epic level), but so far I haven't reached it.