ive always drafted larger vehicles, mostly semi's, and not at recommended distances either, as matt stated, if you cant stop faster than a fully loaded, or even an empty, tractor trailer, then there is something wrong with you or your car.
as a long time motorcycle enthusiast, its also why you >NEVER< tailgate a bike, because they have about 1/2 the stop distance of you in most situations.
I see, on avg, a solid 20-30% benefit from drafting semi's. but the vehicle also plays a role, as in I saw a much larger increase with my old ranger than I do in the mazda protege5 I now own. although I do see a pretty decent increase with the protege
and its quite easy to gauge the "pocket", if your being severely buffeted then your not in it
for cross windy days, try being in the adjacent lane on the opposite side of the truck from the direction of the wind (IE if your heading N, the wind is blowing from the E, stay in the lane to the trucks left side, put your nose about at the end of his bumper)
but make sure to allow for faster traffic, if you need to slow down and change lanes to allow people to pass, do so.