||to be quite honest, I'm not sure anymore. I think it factors itself out. You end up increasing your submission accuracy (slower, less "jump-to-conclusions", more thorough solutions), at the cost of not submitting so often. For low-value contests (250-400-900 type) caffeine is a definite benefit, because of the speed needed and the requirement not to over-engineer the solution. For high-value contests (300-550-1100 type) it's exactly the opposite, and you'll want to avoid caffeine. I'm not sure on challenge ability with/without caffeine yet, my self-study is not yet conclusive.
Man, I've been doing this too long. :)