I just have to smile every time I use my Orbital Super Sawzall. It runs so smooth, I have to check to make sure it's plugged in.(not really but almost)It has the new METAL quick-loc blade clamp. A vast improvement over  the original. As another reviewer already mentioned, there are more  positions on the adjustable shoe. You get two varible speed controls. One  on the trigger for careful cutting, and one on a speed dial  for cutting  different materials. If you run full speed on steel or other hard materials  you will burn your blades up. With the 6521 you can dial it down to nothing  if you want to. Basically it's a super sawzall on steroids. You get more  power,plus the only infinitely variable orbital action I know of. Believe  me, it works ! I have had orbital saws from other companies in the past. I  sold all of them to make room for something better.  I needed something  that could be run continuously without dislodging my fillings, and making  my hands numb. So I got the super sawzall.(Reviewed it in November) I love  it. But sometimes I need more than one, like when I'm on multiple jobs. I  thought about getting a cheapie but resisted the urge. Boy am I glad. The  6521-21 exceeded all my expectations. Obital action with no vibration. Plus  they retained all the good features of the 6537. Including the impact  protecting clutch,and a LIFETIME WARRANTY two things you won't get anywhere  else.  I highly recommend the 6521!