I use them on work-related TPACs--I'm lumping EWR-DEL into the mix--where I know I'll have to hit the ground running upon landing. Been 1K for two years now and have only missed one GPU (TPE-SFO back in April. Ended up sitting in the exit row and having a perfectly nice flight). The rest have cleared, with about 30% clearing at booking and the rest between a month and 24 hours out, with one GPU clearing as I was sitting in my coach seat prior to departure (there are few more satisfying feelings than being plucked from your seat in the back of the bus before a 13-hour flight and taking your place in J).

I typically buy W fares, which on the routes I fly cost about 20-30% more than K fares.

I've never used a GPU on a leisure trip.