Brand certainly makes a difference. Our local farm supply carries KT Industrial rods. They're mostly junk from what I've found. I used to really like their 6013s when they came in a black tube, a few years ago they started coming a yellow or red tube, those suck in comparison. From my research, KT is essentially a reseller and changes suppliers pretty frequently. Their 6011s behave more like a 6013 to me, penetration is a little weak, very thick slag, and they really don't like dirty steel. I still use quite a bit of their 1/8 6011s for thin material though.
I've also noticed that cheap rods are bad about the arc burning up one side of the electrode, like the flux is inconsistent. May be a moisture thing, but I haven't had that problem with the higher quality rods.
My favorite is probably the Lincoln Fleetweld 6011s for general farm stuff. Very little slag, good penetration, don't seem to be bothered too much if the material isn't perfectly clean. Gotta order them online though, nowhere to get them around here. I've also used some ESAB rods before and liked them.