Using screw mount lenses on K-mount body

[Pentax]

What do I need to know about using a screw mount lens on a k-mount body (MX)?

I know you need an adaptor and there's some difference in metering. 
"Stop down metering" which I don't quite understand.

In general does this work out pretty well or is it a pain in the a** 
and I should just look for K-mount equivalents of the lenses?

Some of the ones I want seem to be much more abundant (and 
inexpensive) on e-Bay in their screw mount forms.

Thanks,
Rob