Ray - 0 problems (accept that 15 wanted to default to meta 2). I've done a number of PSQL upgrades and they always seem to go very smooth. This is probably the biggest jump I've done from 12 to 15 and it always just works without an issue. They have been running it now a couple of weeks with 0 incidents. If you have any specific questions feel free to let me know, I'll be glad to tell you if I know something.
Mark