I'm currently running 4e. I've been running 4e non-stop for "other people" since it came out. I've been running it for my regular group since last summer. I'm extremely happy from the DM side as it allows me to be creative, and change things around with very minimal effort.
4e has some warts, but I've found it rather easy to smooth them out without significantly impacting other areas of the game. 4e can also do low magic wonderfully because the underlying math can be adjusted to not use magic at all.
I've run 4e at all levels from 1-20. Heroic, feels heroic and I can easily up the ante to challenge the players. Paragon is heroic on steroids, and it is very fun. Though it does take a bit more to challenge the players. The big difference for me is that in Heroic I was more lenient with hammering the party. In paragon the gloves have come off, and I'm actively pushing them all along. They love that. The game "feels" different because the campaign I've designed is different at each tier. I've used the guidelines on the DMGs to create a campaign which goals grow from the local, to the world, to the multiverse. NPCs significantly change their tune from when the party was heroic, to when they are paragon. Epic will be even more different.
The DMG and DMG 2 are great "tools" for explaining this but fail to hammer that message home, simply making it a mentioned item. It doesn't help that WotC's adventures failed to capitalize on that too, simply making each tier another series of combats. I wish they would have done better in showing the differences. However, since I know that is what I'm supposed to do, I've implemented it and it works fantastically.
I'm working on how to do Epic, since this campaign is a continuation of our long running 3.x campaign. That campaign (3.x) ended with the "ascension" of the previous characters in an act of self-sacrifice to save the world. So my plan is to have the new characters meet the ascended characters who are now exarchs of the gods. It will be a while so I have time to think about it, but I'm sure the players will have a blast with it.
So I'm very happy with my current edition.
