In the beginning, it will really 'perk' them up. Especially in the morning. Over time though, they will get hooked on it. They will be grumpy as hell until you give them more. They will become jittery, and you'll see their leaves quiver for no apparent reason. Avoid anything foo-foo, containing the words latte, mocha, etc. Sorry, couldnt resist.
Ive heard of it, but not used it.