I use soft, flat-ended good-quality brushes from an art supply store (Michael's) and have generally had pretty good results. I am just starting out again, so I don't have any "old" brushes yet, so I just my normal paint brushes (usually the slightly wider ones). I usually choose the specific width of the brush according to what I am drybrushing. Most of the errors and mistakes are the result of operator error, rather than equipment failure.
I gave the el-cheapo Testor's brushes a try for drybrushing after cutting down the bristles a little, but they did not work very well at all (the brush is a little too stiff, doesn't hold the small amount of paint needed for drybrushing very well--resulting in a blotchy drybrush job, and the the bristles seem to be too thick).