what's it like flying in a straight line?
Boring, eventually. At least, after having done an aerobatics course and some competitions (and of course some glider flying), straight and level flying doesn't have much appeal to me anymore. It's become more of a method of getting somewhere (where I need the plane) than an aim in itself.
Oh, and in powered flying if you look backwards you can just see the burned remains of your euro/dollar/pound bills exit the exhaust.