Being sponsored by Rockstar Energy Drinks we sell an absolute shit ton of these, some are zero sugar some aren't.
As with most things, caffeine plus sugar, or caffeine alone will effect different people differently. There's absolutely a time and a place for them, me personally if it is a class on the evening after an ultra busy day then a Rockstar is just the ticket to keep my mind on what I am doing on the mat- we all know the feeling of just not wanting to be there after a mentally challenging day; yet conversely for me personally too much caffeine on an empty stomach can be a big mistake and make me not want to train at all.
On the scale of things we have many guys like Mick our coach who seems to be able drink them like water without any negative effect and conversely one or two who for whatever reason cannot even be in the same room as caffeine without developing some sort of sleeping disorder.
Horses for courses. I would always make sure there's something lining my belly before downing one, but as a very health savvy gym operator would never discourage someone from buying one; otherwise we never would have affiliated ourselves with a company like Rockstar.
On the whole easily a good 90 plus percent of guys buying the drinks feel the benefits of the mental focus rather than any negative side effects. We always stock sugar free to give that choice especially to fighters getting their weight down but even with the sugary varieties as long as its part of a well balanced diet on the whole I see no problem with them. Hope this helps!