Lots of Control - pro
Never tried the combo myself, but that is all i hear about it.
Pro to Dark/Dark:
Awesome stacked with awesome and some awesomesauce on top. Full of win.
Solid AE damage (2 dots, nightfall and TT)
Amazing control.
Good add to any team.
Cons:
Haven't really found any. PVE, an AMAZING character.
Dark epic pool: I really don't like it since my darks usually stay at range for their cones.
Pros: <ul type="square">[*]Excellent controls[*]Excellent AoEs[*]Thematically synergistic[*]Flexible and fun to play[*]The best AoE Stun That is Also a Rez in the game[/list]Cons: <ul type="square">[*]Slow soloing against enemies that resist Negative damage[/list]Definitely worth trying out; they're a blast.
Arc 55669 - Tales of the PPD: One Hell of a Deal (video trailer)
Arc 64511 - The Wrecking Ball
Arc 1745 - The Trouble With Trimbles
Arc 302901 - HappyCorpse
3xd is fun and can with a little know how can tank 8 man spawns
To hit debuff, slows, resistance debuff, regen debuff, damage potential debuff times 2, soft controls and immobilizations, a super pet, cones and AoE action- everything a potent support player would want, lacking just one thing, defense debuff.
May I also suggest, as is my usual vent: take howling twilight, slot it for recharge and use it often. Sometime you might actually rez a team mate.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
Previous respondents have said all that really need be said, but I'll add a couple things anyway (being a wordy sort of person...):
* A great defender for players who don't care to spend a significant portion of their time renewing buffs. 'Coz there aren't any...
* The Swiss Army Knife of heroes. You can contribute on so many fronts that sometimes the greatest challenge in playing a D3 is prioritization.
*The combination of Darkest Night, Fearsome Stare, and Tenebrous Tentacles can put an entire spawn at the ToHit floor. Sprinkle with similarly debuffing blasts, and you can function in the tanker role. Seriously.
Learn "snaring." For dangerous spawns that your team can't just steamroll, snaring constitutes "team godmode." Let me know if you want a snaring primer here in this thread.
Make one. Trust me. Best AT in the game...
"And in this moment, I will not run.
It is my place to stand.
We few shall carry hope
Within our bloodied hands."
I played Dark/ and /Dark but not together. But both are great sets that do work great together. Lots of 'Pros' that have been covered already.
But just to mention the villians that have negative energy resistance are a royal pain. Pantheon, Banished, etc. On the plus side of that though Rikti dont have resistance to it and on things like Rikti drones Negative energy blasts just blow them up fast.
DDD, better controller than many controllers. At least until you start arguing the semantics of what "control", "many", "some" and other words mean. Then 137 pages later you're so confused you just don't care anymore.
I disagree with it being the best AT in the game. It's the best AT's in the game. :P.
You have so many tools, you can fill any role in a team, damage, control, and tank or just straight out debuff. The best part is you can usually fill any 2 or 3 of these roles with one spec. 
I pretty much solo'd from 1-50 in 3 months playing 2-4 hours a day doing missions and streetsweeping only, no TF's, Farms, or PLing. I did not find any villians overpowering.
D3 is the Jack/Jill of all trades, the set lets you accomplish just about anything but is not as powerful as a specific AT. You will have massive damage but not on the scale of a scrapper or fire blaster.
You will have bigger heals but you need a villian target, something a Emp doesnt need, same with Rez.
My Dark Defender and my Warshade (for when i feel like shooting everything - squid form) are my favs 
You can solo with no problem, and any team you join should be jumping for joy cus they're going to just streamroll the mobs from that point on.
I always give a cheer when I see the team I'm on has a D4.
Yes, they do make that much different on a team. If fact, I always up the mission dif just for that very reason.
/Empaths can turn three people into Jesus, one person into God, and everyone else into the twelve apostles.~Angry_Citizen
Don't you know that discussion of power selection/slotting can ONLY be based on hearsay, rumor, idle speculation, and bald-faced lies??!? ~Elf_Sniper
I know I could probably search for information on this combo but I'm wondering what the pros and cons for this build are.