I'm one of them.
Here's the scoop:
The Clarion workshops are a sort of writer's boot camp. They're not for everyone -- the criticism can be intense -- but for those who are willing to live, eat, drink, and breathe SF and fantasy for a summer, there's no better training. Admission is pretty competitive as well. Many apply, few are chosen.
But over the decades Clarion and Clarion West have turned out more professional SF and fantasy writers than all the college creative writing programs in the country combined.
Scholarships are available for those who cannot afford the tuition.