PGO is limited if you are chasing throughput on really tight programs that spend most of their time in a small number of loops (preferably with textbooks written about them), but it's really useful if you have a bunch of branchy code that mainly does one thing but has to cater for other things too (compilers come to mind).

Enabling PGO when building the D compiler made it almost 50% faster on some parts of the test suite (and fairly reasonable big files at that) - speedup elsewhere was not as dramatic but still a tidy win.