IMO, it's exceedingly excellent. I have been running a Dark Sun 4e game since the setting came out a year ago, and it's just going incredibly smoothly.
The fluff is great - it reset the timeline to right after the events of The Verdant Passage, so Tyr is free and Kalak is dead, but that's about it. Where you go next is up to you. (In my own game, after some adventuring, I'm running them through the 2e adventure, The Road to Urik. It's going awesomely.)
I've used both Eladrin and Tieflings to some great effect in my game. I think they did a good job on the fluff, there. Not a single dragonborn has been encountered yet. In my game, I've made Goliaths a separate race from Half-Giants... Half-Giants are Large-sized brutes, much like in 2e, but are not a PC race.
Overall, as a huge fan of the 2e setting, I'm super-pleased at 4e's interpretation of it.
-O