It depends on the type of coffee you are drinking.
Nescafe, is good if you want to start jumping around like a jack-in-the-box.
The purpose of cafeine when working out (appart from energy) is to change the fat/energy burning ratio towards more fat burn.
Nescafe is bad quality coffee , it has any number of other chemicals in it, thats why if you put a litle water and shake it froths up. Do you think that normal ground beans froth when you shake them??
Get a good coffee machine, get a good cup of coffee, dont drink to much and you will be fine.
I love coffee, Its the life juice