From what my endo has told me, persistent hypos can be just as bad to brain function. Hypos also interfere with brain development, which is why the have diabetic children "run higher." I don't typically feel symptoms of a high until I'm around 220 or so. Even then, the symptoms can be vague - a strange pressure in my head, blurry vision, a dry mouth. Once I'm over 300, however, the symptoms become very obvious and nausea sets in. For me, a day with lots of hypos interferes more with my ability to work and get things done. A day with mild highs doesn't bother me too much, but highs in the upper 200s and above make me just want to go to sleep. The only good thing about lows is I can generally treat them fast. Treating a high is trickier because the amount of insulin I need varies and I'm always a little nervous correcting.