EDIT: I'm speaking about free DLC that comes with Gold/GOTY/WTF Editions. I've never paid for DLC in my life. My idea of Good DLC is the one you get for free in the first place.
The first Borderlands did DLC right. New missions, new enemies, new challenges, new locales, new characters, etc. "The Zombie Island of Dr. Ned" is like Borderlands' Halloween Special and was a lot of fun. "The Secret Armory of General Knoxx" and "Claptrap's New Robot Revolution" were a lot of fun as well. "Moxxi's Underdome" not so much, it's basically a battle arena and an incredibly taxing one at that, too. The harder levels take up something like 2 hours to complete and there's no saving at any point :/ But the other DLC was a lot of fun and felt like a fun bonus that extended the experience considerably.
Assassin's Creed II had "Bonfire of Vanities" and "The Battle of Forli". I liked them both, each added two self-contained, historically sensitive narratives that blend in seamlessly into AC2 while working on their own as well. I especially liked 'Bonfire' since it was all about stealth and assassinations (and Savonarola makes a great villain), as opposed to the action-oriented 'Forli'. I hear there's another DLC for exploring the Auditore family crypt, but that it boils down to a bit of tepid platforming.
Resident Evil 5's DLC was OK. "Desperate Escape" is pretty intense on any difficulty. "Lost in Nightmares" is basically a taste of Revelations. I liked it a lot, it reminded me of good old RE days and was the closest thing the game ever got to being scary.
On the other hand Far Cry 3 has mostly crap DLC. So far I cashed in the following: a wallpaper, a revolver, a tattoo skin pack and a mission called "Testing Unit". I timed it. It's 13 minutes of walking around an abandoned lab. You shoot a bear, 2 snakes and about half a dozen dogs. You get 2000 XP and $2000. That's it. No mileage, no narrative, no challenge, no difference. I hear "Monkey Business" is good though. And it's nice they extended the co-op campaign by 2 more chapters, at least on the PS3.
Batman Arkham Asylum doesn't have good DLC. I don't care for more challenge maps, they're all more or less the same. Playing as Joker in challenge mode is a fun novelty for a few minutes, and that's that. The combat system was designed for Batman. Joker controls poorly, especially in the stealth maps. Batman Arkham City did DLC a bit better, blending in Catwoman into the main story and adding the "Harley Quinn's Revenge" epilogue, but that - while fun to play - was ultimately disappointing. Robin controls differently enough from Batman and feels like an upgrade on some counts of combat (stealth, not so much). But the story goes nowhere. After Arkham City ends on such a strong note, Harley Quinn's Revenge plays like an unnecessary afterthought.
I don't care for downloadable maps/skins/weapons/wallpapers. That's the kind of stuff that shouldn't get cut out in the first place. And if it does, let me get if for free. What I like are extra missions/campaigns. Whether they're bonus stuff or they blend into the main story, I don't care. Either give me more of the good stuff (assassinations in AC2, scary survival in RE5) or carry on doing whatever it was that you were doing in the first place because that just dandy, no matter if you're continuing the main narrative or making up something entirely new and self-contained (Borderlands).
