Besides what others have mentioned (though that's not recently been much of an issue for me personally) it often makes me sweat a fair bit more than normal (though I don't think I'm any smellier as a result - so it's not really a negative thing, I suppose) and it impairs the regulation of body temperature, which isn't normally a big deal (I find the occasional goosebumps quite pleasant, if anything) but it does tend to make me feel a bit hot 'n' bothered if I'm doing stuff out in the sun on a hot day - it sort of feels like I would be somewhat more susceptible to heatstroke, though I've been quite careful to avoid that, so who knows..
During the first month or so of daily use (80mg/day, mostly taken in the morning, never later than midday), I experienced a few other negatives, though I reckon they were mostly knock-on effects from lack of sleep. I didn't suffer insomnia as such (I got at least some sleep every night) but I was probably getting an average of an hour's less sleep than normal every night which obviously stacked up over time and did go on for a fair while. As my sleep debt increased I started to feel a little cracked out at times mostly, if not exclusively, late in the evening as bedtime approached. I would quite often falsely sense movement in my peripheral vision or, the most the annoying one, be pleasantly drifting off to sleep and suddenly hear the sound of a fly or bee or something zooming towards my face, which would make me jump and snap my eyes open (there was never anything there, of course). Eventually, though, the lack of sleep caught up with me and after a brief phase of oscillation between feeling wired and cracked out in the evenings and crashing out with ease, a sort of equilibrium was reached (if memory serves, there was some variation but each full cycle lasted about five or six days and I felt more or less stable after two to three weeks.) Nowadays, I only get the side-effects mentioned above and, if I overdo it a bit, pilly-willy.