Although I can't speak for the disgruntled, I've been using ShapeShifter since it was first released and theme-ing my Mac since before that. ShapeShifter makes life easier if you want to vary the look of your desktop and work environment. Many themes are a bit "too much", but there are several that simply refine the Aqua interface in a way that suits me. I can think of several themes that would likely satisfy users with simpler tastes, but when the claim is made that these themes "can't" be found, well, the effort hasn't been put forth. Search Google for "themes + Mac" for starters. I would never say that this speeds anything up for me, but it certainly doesn't slow anything down either. Good software, good company.