We playtested 4e last night with paragon level characters (i.e. 16th level) using paragon paths, or otherwise paragon multiclassing.
The biggest impression I got from the game is that it was a real grind. Combats lasted too long and besides never really having the feeling of being in danger, for the most part the game has too many hit points and not nearly enough damage. I also felt very straight-jacketed by the "roles" (I played a wizard/blood mage so therefore I was a "controller"), and IMHO really detracted from my enjoyment of the game (in previous editions, wizards/mages could in fact be either "controllers" or "strikers," a flexibility 4e lacks IMHO). While there were a few interesting abilities for the Blood Mage paragon path, it felt very watered down with not a whole lot to really make the paragon path stand out and be unique. Furthermore, all the abilities really started feeling the same, since it was more often than not a variation on a theme. There were a few really good ones (prismatic beams), but many were very decidedly "meh."
Also, while 4e really streamlined the core rules, it tossed any sort of idea of combat streamlining by piling on the special effects, ongoing effects, instant interrupt effects, etc. The amount of added in stuff was IMHO astounding, and I really can't see how combats in this game could be faster than 3e combat. Perhaps part of it was our character builds, but I couldn't really see any builds that made things simpler. I think the most telling aspect here was from the DM, who pushed for and organized this playtest. After the game he outright declared he is "done" DMing 4e.
And I think I'm done playing 4e too.