Buying out of teaching - opinions?

This is a topic that comes up at many research universities, and I'd be curious for your opinions.  Some institutions formally allow researchers to "buy" out of teaching responsibilities.  Some places actively encourage this practice, as a way to try to boost research output and standing.  Does this work overall?  Faculty who spend all their time on research should generally be more research-productive, though it would be interesting to see quantitatively how much so.  Of course, undergraduate and graduate classroom education is also an essential part of university life, and often (though certainly not always) productive researchers are among the better teachers.  It's a fair question to ask whether teaching buyout is a net good for the university as a whole.  What do you think?