I played in a campaign that took my wizard to 20th level and it was a lot of fun. I think it worked well because the emphasis was not on combat every game and it was abundantly clear that there is always something MUCH tougher out there in the game world. There was a continuum that ranged well above and below the characters that was relevan. Also in the campaign, doing something in character and in flavor was more effective than stats so much lower level characters were useful and good at their roles/niches.
It is a lot of fun to run a high level character in a well run campaign. You get to use a wide variety of spells and tactics that can only be utilized by powerful characters.
On the other hand, the campaigns I run tend to be low to mid-level (2-10) games. I have a general feel for the power balance at these levels and can introduce elements and predict how things will generally turn out (tough but doable fight, easy encounter, etc.) The players generally feel very powerful and sometimes heroic but since I run mostly horror, there is usually a palpable sense of danger.