I ran two short campaigns of D&D 4th Edition in the Summer of 2008 (when it came out), to see what it was like and give it a proper go.

After the first campaign, running through the Keep of the Shadowfell (levels 1-4), we were still fairly positive. We'd enjoyed it, and although some parts felt slightly strange, it had been the first time we had encountered the system, with pre-made characters and a pre-written module (not always my favourite).

For our second campaign, my players generated 15th level (Paragon tier) characters and I ran a campaign in my Homebrew, which had been updated to reflect the default 4th Ed assumptions (Points of Light, Empires that were mentioned, gods available).
 They had an interesting party of a halfling warlock (later replaced by a minotaur warlord / paladin), tiefling wizard, dwarf cleric, drow rogue, bugbear fighter, halfelf cleric and dwarf ranger. They were fairly well organised and worked reasonably well as a team.

I found the game fairly easy to prepare for (quicker than 3.5), and experimented with new monster creation (an elite ettercap and a copper piece golem). That was all good.

However, as the campaign proceeded, we found the game ground down occasionally, and certain parts of it seemed to be very artificial. Some of these parts seemed to be there for meta-game or arbitrary reasons and not as elegant representations of the game world. With two clerics and some other healing abilities, the characters also rarely felt truly threatened.

It finished off with an epic (in length) fight with a beholder and a couple of its allies, which ground out for many rounds. At one stage, a raksasha assassin spent three rounds trying to coup-de-grace the unconcious dwarf ranger without killing him (as he healed from various other effects each round). The fight finished off our desire to play any more 4th edition for the time being, and we moved on to Dark Heresy and Savage Worlds.

One day I may play in a campaign if someone was willing to run, but my D&D campaigns will probably be taken care of by True 20 by Green Ronin or Pathfinder by Paizo Publishing in the future.