Story Time:
I've taken a number of supplements. I've taken Jack3D mixed with more dilators, I've taken almost every PWO supplement know to man. Its been intense. Nothing has given me a better feeling than Muscle Pharm Assault. I've stuck with that for the last 2 years.
With that being said, I've been PWO clean for a month now. Why? My cousin ruptured his heart taking PWO+Dilators and working out immediately. Since Caffeine is such a vasoconstrictor, it counters the increased pressure and flow of blood through the veins. If this pressure is too much, it'll tear vaso-tissue. Imagine squeezing your intercooler tubing at full peak boost.
He now has heart murmurs and had to have heart surgery to fix it. He will be fine, but the scare was enough for me quit all supplements. (Even protein, I get it from actual food sources now). He took low doses, too. I have an occasional 16oz coffee in the morning and workout in the afternoon, but nothing like drinking it before I work out.
YMMV, but its something to consider.
In fact, one of the best things you can drink (Minus empty calories) is a beer. Its straight carbs and a vasodilator!