I've used them on a couple of occasions and was impressed. Not enough to buy yet another set of lights, but wouldn't hesitate if I needed them.
I have to observe that you may find 600's to be limiting power-wise for some settings, particularly if for larger spaces or groups of people, or especially when you need lots of DOF. It doesn't fit everyone, but our rule is you seldom have too much light power but when you need it most, it's easy to have too little. We have some 800 w/s blocks that work okay in our smallest shooting room, but generally don't leave the shop with anything less than 1600 w/s. YMMV.