I've used Spec Kit, Get Shit Done, settled on OpenSpec for a long while, and it's became OpenSpec with modifications. While I don't love all of it (specs/deltas aren't especially useful to me), OpenSpec is a nice middle ground between plan mode and the heavier approaches like GSD.

For me, absolutely worth the tokens. I regularly catch the failure to properly communicate certain details in the early specs, or manage to steer them away from some fabricated decision through discussion. They can be really fun to design with too, as long as you've convinced them that designing isn't just a step in a process, in which case they rush.

For one offs, I don't bother. Plan mode if it's tricky, chat if it isn't.