My first encounter with Smith was in an old Judy Merril anthology with "A Planet Named Shayol." It knocked me sideways---and I was far from being an infernologist at the time!
Agreed: start with the mature work. Don't forget "The Dead Lady of Clown Town." Or even "Mother Hitton's Littul Kittons." Don't read those Casher O'Neill novellas first as the Rat suggests---they're for after you know the score.
-alga-