I think the 3e sweet spot is 4th level - robust PCs who can't wipe out a horde of gnolls with fireball.
Re 4e, at the London D&D Meetup, whenever a DM suggests running a Paragon-tier game, IME the reaction is: "It's too complicated! You should run at Heroic Tier instead". The one exception is the campaign that has organically gone from 1st to high-Paragon, so the players and DM are familiar with PC abilities and how the game develops.
However, I think that 4e Paragon now is a lot more playable, with the simpler Essentials characters, and the newer monster stats that do more damage and inflict fewer status effects.
Re 4e, at the London D&D Meetup, whenever a DM suggests running a Paragon-tier game, IME the reaction is: "It's too complicated! You should run at Heroic Tier instead". The one exception is the campaign that has organically gone from 1st to high-Paragon, so the players and DM are familiar with PC abilities and how the game develops.
However, I think that 4e Paragon now is a lot more playable, with the simpler Essentials characters, and the newer monster stats that do more damage and inflict fewer status effects.