Dark/Dark is a good Scrapper, though pretty much every combo is a good Scrapper, so that may not mean much. If you want to split them apart, I'd recommend Katana/Dark (good survivability), Dark/Shield (DPS) or Dark/Invuln (max survivability). I'm not sure what bad things you're hearing about the sets. They have drawbacks, of course, like Dark Melee's poor AoE and Dark Armor's hefty endurance consumption. Both sets also also work best with some investment, but that's true of most things.
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I've always liked Dark armor, because it's so very well balanced between resists, defenses, and healing. It provides you a lot of exotic and resists that you just don't get from many other sets, such as psionic and toxic.
I have actually never really given Katana a serious try until I saw a discussion that Werner was having over a Kat/Regen build. I ended up giving Kat/Dark a try and in doing some research on that found Werner already had a very successful Kat/Dark build and while quite expensive turns out to be well worth the investment.
I've nearly completed my Kat/Dark with just a few HO's and a single purple missing from my build. Already, my survivability is extremely high in both regular and incarnate content. The only issue has been endurance consumption which becomes far more managable when you get cardic alpha.
Divine avalanche always you to fill in a gap that you normally would be able to build for and allows you to bolster other aspects leaving your build quite balanced. Even exemped down, Kat/Dark has remained a potent combination. I find that I run much less of my toggles at lowever level, but dark regeneration is probably the most potent scrapper healing ability in the game when you are able to get a decent level of saturation on it.
I, too, highly recommend Katana/Dark as a more than viable Dark scrapper. All of Werner's suggestions seem to be the most popular and well thought of Dark combinations.
Look for Dechs Kaison's post(s) on Dark/Dark. He creates builds Scrappers that Tank, yet kill like Blasters. That's maybe a slight exaggeration, mebbe not.
On a more pedestrian mode I am leveling up a Dark/Invul scrapper. Just hit 32.
One build is pure sewers build. With extra ttacks from Flight and Fighting and 6 slots in all attacks, all available during the trial. Nice attack chain available that way.
My other build is how I usually build, and really not optimized, and I am very aware of that. All SOs, and the slotting is to my preference, but I am sure it could be better, even with SOs.
So I just did the Respec trial, with a MM, a Peacebringer and 2 Tanks. I out tanked the tanks, and was lead into 90% of the spawns, and of course I'm doing a ton more damage than the tanks. My only complaint is the damage over time aspect of the cone, but I still like it. Since I have the same cone for the vet attack, when I line up a sweet mob(s) I can hit them both and really grind 3 guys at once. I only hit Dull Pain 2 times during the whole trial. One time was because the team did not show up for 2 minutes on one of the spawns in the reactor. It was late in the trial, I was out of purps, and had to use Dull Pain and Siphon Life to stay up. Again, all with SOs, soloing the whole spawn. I did not hit 32 until the end of the trial, so Midnight Grasp was not in the chain. More damage over time, but at least it 1) has character 2) is big damage 3) has some control in it.
I have run a Dark/Dark brute thru the incarnate trials. 95% only SOs. He has 3 -KB procs and a +End Proc in the AoE attack heal. Thats it. He is a monster. I love to throw cash at a main, but he serously just doesnt need it, freaking mean with just a tiny amount of love.
I've quite enjoyed my KM/DARK, which I have plans to softcap S/L Defense with. I think you'd be fine with Dark Melee/Dark Armor though, if you're willing to rely on a bit less AOE (and as a Scrapper, there's only a few sets that really shine in the AOE department imo).
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Yup, another Vexen talk on powers. Me and my altaholicness...but anyway I'm focusing now on the Dark powers, all of them. Almost everytime I have done a dark related power-set, it usually goes well.
Necro/Dark MM- 36
Dark/Dark Def- 26
Now I can make a Dark/Dark Blaster, so while I was lvling him up, I pondered why I couldn't stick with a Dark scrapper/brute. It's not the fact that it is a close range fighter, (As all my 40s are Brutes), but I just can't get into it. Plus I always hear bad things about the sets, so now I ask:
Should I try again at a Dark Melee/Dark Armor? (Advice be nice) Or should I try different combos for the two dark powers?