I dirnk lots of red bull because I like it.
Anyway, to answer your question, sometimes I have one just before lifting if I am feeling low in carbs (you know, that fuzzy-head feeling you get from eating less carbs than usual) and I want a caffeine-and-sugar "lift". I am not exactly lean compared to most folks on here so one drink isnt't going to make a huge difference to body fat.
Problem is, you need to make sure your workout finishes BEFORE that lift departs as you tend to be worse afterwards than before (sugar rush - insulin crash).
So, if you just need it to get going, and aren't worried about numbers, or total carb intake, why not, but if you are a high-level bber, worried about carbs or plan on doing a long workout and care about the numbers after the first 20 minutes, don't.
OJ is almost as good with a slower glycaemic "slope" in lifting up or down.
YMMV of course, if it works for you, go ahead, if not, don't. Try it and see.