Any admins have quick comments on Applications Performance Management? Is it a “you need to be using it” tool, nice to have, or not great?

I'm not an admin, but I vote for 'you need to be using it'.

it's free, also if your instance have lots of custom scripts then this is a must have module. Most of all it is FREEEEEEEE

yes, make sure you install from SuiteApp Marketplace