Well - in a nutshell, the slower the speed, the less torque you will have. That sounds pretty obvious and you already know that. You can run the motor pretty damned slow, but you will be able to stop it with one finger so it's really basically useless for doing any work. The nice thing about the arrangement is that if you try to keep the motor in the top 75% or so of it's RPM, you can adjust to suit conditions - on the fly.
I have a VFD on my Bridgeport, and I run it from very slow speeds for tapping all the way to max RPM. The nicest thing about it is you can get the speed close with the belt arrangement and then fine tune it as you cut.
Yes - the motor does get a little warm at low RPM.