Here's a couple articles.  Some recent research seems to indicate it's a bad idea.  Other research tends to indicate a performance boost from moderate amounts of caffeine.

http://www.medicalnewstoday.com/articles/55664.php

http://www.runnersworld.com/article/0,7120,s6-242-301--13105-0,00.html

Len

I'm addicted to caffeine.  I can't remember the last morning I didn't have coffee.  It had to be 30 or 40 years ago.  My last physical the doctor told me how healthy I was  ...  marveled at my cholesteral levels, etc.  ...  but had 1 issue.  She thought I was consuming too much coffee  ...  2-4 cups a day.  I chuckled.  She had misread my handwriting.  It was 24 cups a day. 

Last year I toured a coffee plantation in Costa Rica.  For whatever reason, I now only drink 2 cups a day.  Don't kid yourself  ...  I crave it all day  ...  but limit myself to only 2 cups in the morning.

I have never run a marathon WITHOUT coffee.  Never will.  My GUs have caffeine in them.  Caffeine is good  ...  at least for me.  Positives  ...  negatives  ...  I just don't care.  I'm an addict.

Ceasar  --  is that your Xmas card pic?  You could have communicated your point with words instead of   ...  that.

Yes I could have communicated my point without the picture ...

It's actually my motivational poster ... - that's dedication right there - the courage to keep going, even when you have to go ....

HardCoreTrainer summarized perfectly.

I drank coffee before a race once and had a great run.  So for the next race I figured if I had a lot of coffee I'd do even better.  No way... I didn't do as well as I expected and I felt awful afterwards.  I was noticeably more drenched with sweat than usual which meant that the excess caffeine caused me to dehydrate more than I normally do.  I still like to have some caffeine in me before a race, but only in moderation.

As far as the guy in the pic, the poor fellow threw up.  It should be obvious because it's on the front of his shorts and front of his legs.  No bowel issues with him.

I am glad I saved that pic before it was removed. It was gross, but very motivational at the same time. I hope he won the race.

I have an interesting and different take on this post:

I have recently given up all caffeine...  because my doctor suggested it might be

contributing to my recurring sinus infections.  I know, it sounds strange and I balked him

for some time. But here is the scoop:

Actually caffeine and peppermint relax the valve at the end of the esophagus and allow stomach acid

to mist up.

This misting irritates the lungs, sinus, eustachian surfaces and makes them

susceptable to infections.

Since giving up the caffeine, I have been able to quit taking Claritin on a daily basis

and have not had the reoccurring infections that generally hit me during October and November.

My running has helped me lose weight which is also helping with the acid reflux.

I used to drink 4-5 cups of coffee and two cups of sweet tea daily.

I am so greatful for the improved health.

Just my two cents worth...