I did quite a bit of drafting while driving cross-country, and it definitely helps, especially when bucking a headwind or driving at interstate speeds (70-80). I only did it on the open highway, and didn't follow too terribly close (which is a subjective distance, I admit). I've also actually tried it on uncongested highways in regular metro areas, with great success... it is somewhat addicting watching those MPG numbers climb so fast.

You can really feel it when you get into the right area, the ICE just drops its RPMs and everythign feels relaxed, as it would if you were driving w/o drafting at 20MPH less. On the open road, some trucks don't like it and you'll soon know, because they'll pass another truck and pull back in front if it without allowing you space to squeeze in. Others don't mind at all, and are quite accomodating. I drafted one for a couple hundred miles and he would flash the brake lights to warn me whenever there looked like a slowdown ahead, always gave me room to stay tucked in behind when passing, and when he finally had to pull off at a weigh station, he gave a big wave, smile and blast of the horn as I passed. These are the ones I would look for since we could work as a team and the safety was greatly increased.

The most comfortable though was when we had strong crosswinds and the best spot to draft was not directly behind but halfway or completely in the next left lane. This was a nice calm spot not buffeted by the wind, where I could experience the benefits of drafting and the safety of seeing everything down the road ahead.