As all experienced GMs and players can attest, D20 rules scale well on paper but not so well in practice. In both D&D 3.5 and Pathfinder, campaigns run smoothly for at least 10 levels, but by level 15 play starts to slow down, and by level 20 a single combat might take an entire play session to complete, largely due to the sheer number of spells, feats, abilities, resistances and magical effects in play. Spellcasters in particular might have a plan of action all lined up, then just before their turn it's discovered that oh - that creature is resistant to (whatever), and they have to spend five minutes digging through countless spells in the hope of finding one that will work.
We've tried epic level play, though quite honestly the D&D 3.0 ELH really didn't help matters much - obviously it wasn't thoroughly playtested prior to publication - and we had to halt our epic campaign by level 22.
Does Paizo have a timeline on producing an ELH that will let campaigns go truly epic? In my mind, this is the single greatest need that has remained unaddressed since day one of Pathfinder.
Personally, I have long felt that the key to keeping the game playable would be making it so that as epic PCs level up, they can "swap out" groups of spells or feats in exchange for single, more powerful/flexible epic level abilities. The key is to give a clear advantage to each new ability while having it replace enough "non epic" spells, feats, etc., to noticeably streamline game play.
I am hopeful that the introduction of the Planar Handbook will make development of an ELH a priority, because obviously there is room in the multiverse for campaigns to go to levels 40, 60 or beyond.