Caffeine itself only lasts so long, and you build up a tolerance to it pretty easily. If you take 4 full days off of it and then use it pre-workout, then it will have its full effect again. But regardless, caffeine on its own only lasts so long, and will have you crashing eventually. And N.O. products are total, complete bullshit. The only reason you feel a buzz from them is because they usually contain caffeine! The "pump" is useless -- it has nothing to do with building strength or muscle. Pure marketing BS.
long-lasting, kick-ass but not-overly-stimulating energy and focus, I've found that LightSpeed
is the absolute bomb. It's got a little caffeine, though not a lot, but that's not what makes it get you SO freakin' zoned in and last SO long. My focus and endurance is just . . . totally different when I use it. It's like you're totally motivated to tear through your workout.
I occasionally use just a simple double espresso shot if I don't have LightSpeed handy, but that only seems to last for the first half of the workout. I'll still sometimes drag towards the end if I go the coffee/espresso route. But the endurance/motivation from LS is kick-ass.