I have a Highland Park 5 wheel grinder/polisher machine. I have used it quite a bit for grinding rounded shapes like cabochons or hearts and the like. It is also useful for grinding rough edges off thunder eggs or book ends you want to run on the Vibralap or Flat Lap. There's a 100, 220, and two expandable drums on this that I have a 220 and 600 affixed. These are somewhat troublesome so I removed the leather "keepers and now use a centering guide when I start the machine. Depending on you interests, this kind of machine can be either crucial or optional. My interest in box-making reduces my need for this machine. Compared to the flat lap, it is slow and uneven in terms of larger flat surfaces. Works great on small stuff, so if jewelry is your passion, this machine is for you.
See the videos below.