Pretty good, it's basically a max of 4 Engage energy drinks, or 2 of them and a mix of Nos or Monster. I don't feel jittery at all, but I do get a headache if I don't get any caffeine within a few hours of waking up.

I used to get a little heart weirdness (a weird beat once in a while), but I've cut down smoking to 1 or 2 cigarettes per day and started drinking more water and the heart weirdness has stopped happening.