Dark Sun is awesome.
The Good
-Captures the setting adequately, lots and lots of little places and adventure hooks are mapped out throughout the campaign book.
-City-states are particularly well-detailed, and you get a pretty good sense of the culture of each one.
-Combat is deadly, deadly, deadly. Dark Sun monsters are downright beastly, some of them. If harsh and gritty deadliness is something you consider a prerequesite to Dark Sun, then rest assured they pretty much nailed this.
-Themes are awesome and fun. So good they just had to add them to core D&D, in fact.
-Weapon breakage rules come in two flavors; one is really rough and gives you a desperate scrabbling-for-survival feel, the other is optional and gives you another option in combat, giving the game yet more strategic depth.
-Tight restrictions on which classes and races are allowed (no divine classes!) makes sure that the Dark Sun campaign feels very authentic and true-to-its-roots.
The Questionable
-Arcane Defiling doesn't really cut it for me, personally. You cause half a healing surge of damage to your allies in order to reroll a daily attack roll or damage roll. Defiling otherwise has no effect on the game. This sucks, because what is supposed to be a huge element of the setting (it's the reason the whole dang world is a desert) has become a minor mechanical footnote. A lot of arcane daily powers are either a) big area of effects, in which case rerolling only a single die isn't going to make much of an impact or b) stuff like Flaming Sphere or Wall of Fire, where there simply isn't much reason to do it.
Plus, with DS monsters dishing out higher-than-average damage, and with divine classes (the best healers) disallowed, knocking off a bunch of HPs from your teammates is a pretty good way to start the TPK chain-reaction. Simply put, I have never once used Arcane Defiling in a DS game, or seen it used, because it's very rarely a good tactical choice. So I play a defiler... that never defiles. It kind of stinks.
-YMMV on how the 'new' core races fit into the campaign setting. Personally, I think Eladrin are awesome and fit in beautifully, but Dragonborn (Dray) and Tieflings are just kind of there, busy being Dragonborn and Tieflings. Once again, YMMV.
-Cities feel too cosmopolitan. Some of the city-states have mention of significant minorities of halflings and thri-kreen in them. Personally, I have no idea why the sorcerer-kings would allow races that routinely KILL AND EAT other humanoids into their cities. I personally would've kept halflings and thri-kreen to their respective societies, and let adventuring PCs deal with all the fun culture shock and complications.
-Overall, the campaign book doesn't do a very good job reflecting what I consider to be the tone of Dark Sun; Dark Sun 'adventures' should be more about finding food and water to the survive the next day, and less about plundering ancient ruins for goodies. The sections that detail the setting, outside of the city-states themselves, quickly become list after list of "adventure sites." "This ancient ruins has some giants guarding glowing treasure! This hidden valley is home to a powerful druid! Here there be drakes!" It's great that they're giving us adventure ideas, but I think Dark Sun requires a shift in what you consider an adventure, and this doesn't always come through.
-The 'harsh and gritty' might slant toward too harsh and gritty. I've been playing in a Dark Sun game for 5 levels now. I've gone through 5 characters. To be fair, I was 'doubling up' on characters, playing two at a time to make up for a lack of players. And this is obviously going to change somewhat depending on your DM, but the fact remains... I've seen two TPKs in Dark Sun, and numerous close calls.
There's also an adventure module out there that's rather notorious for being brutal. I don't know what it's called (might ask the DM tomorrow and report back later), but it's absurdly brutal. It's intended for levels 6-9, or something like that, but the first fight is against two bloodied solos. The second fight was against some insubstantial and phasing foes (always annoying). The third was a forgettable cakewalk (for my party, at least), but the fourth was against some reskinned stirge swarms (an overpowered, level 10 creature) and resulted in a TPK.
When you're level 9, and you hear your DM say, "Okay, you're now taking ongoing 40" then you know you're really, really boned.
There was apparently a fifth encounter, which I never got to see, but our DM assured us it was a doozy.
-This is a minor gripe, but I think banning divine classess, while an awesome way to keep things retro, had some unintended consequences. There are only 8 leader classes (9, if you count the Warpriest separately). Eliminating the Divine classes, you're down to 6 choices, and you also lose out on the Paladin, a fairly solid substitute-leader.
It gets worse; the Sentinel has a built-in ressurection mechanic, which many DMs interested in running a harsh-and-gritty permadeath DS game will not like. There's also an issue with the Artifcer: some of their class features key off of magic items, and since Dark Sun is intended to be fairly scarce on the magic items, this is a sub-optimal choice at best. This leaves you with only 4 viable options for your team leader. And if you think Bards are silly, or don't like the power point structure for Ardents... then, well, hope you like playing Warlords and Shaman, buddy.