I have a KM/Elec and I love it. It does feel squishy in some situations (not many, but it is there), though. The extra +recharge is wonderful. I didn't take the damage aura, so I can't help you there, and the -damage meshes with the +Resistance, but it helps out +Defense sets just as well (when you get hit, it's for less, ect.)
KM has a high DPS and the -Damage is nice, so anything should mesh with it well. I hear people enjoy /Fire with it to go all damage, but I've heard good things of both /Invul and /Dark as well. I am currently debating between those two, to be honest ... haha.
All in all, KM is a great set, it grows as you play and can work with pretty much anything you wanted! 
Hey all, just freshly back in after about a year and looking to roll my first KM, just need a solid secondary now.
Only thing I don't want to look at is /shields (animations just don't work for me, shame about that).
So right now I'm wondering how /elec performs, I would think the damage aura would stack well with PS, the recharge would set up some extra primary damage, and the +res would work with -dam...
However, I've never really used /elec for scrappers, actually I've never used the resistance based sets at all for scrappers (save /fire, which worked out great after IO's)
So, does /fire or /dark beat out /elec soundly (no one talks about it on the forums much for some reason it seems like)
Or does KM Have enough damage that things like invuln or regen or somn still feel good?
So far, leaning towards /elec, thoughts?
"Fascinating. I'm not bored at all, I swear." -Kikuchiyo