Caffiene will give you a buzz/lift at the start of training (it is not long term, similar to sugar). It will raise your metabolism (so is great for using while weight cutting). It will not make you gas, what excess caffiene can do is dehydrate you. If you are dehydrated, then you gas easier. If you stay hydrated and have a caffiene drink before training you will be fine. The downs you may feel are probably related to the sugar in the carbonated drinks not the caffiene.
Caffiene is a perfromance enhancer. If it wasn't, it wouldn't be banned by the IOC. Elevated levels can offer enhanced performances.