I've got over 660,000 banked, have had as much as 850,000 but during bankrutpcy I burned down to 350,000 on trips for myself, friends & family. I'm trying to save them for upgrading or free trips during retirement. My CR-1s, 500 milers, & system wides seem to work well for a good portion of my annual flying ( the longer or very early domestic, and almost all international). I realize they lose value over time, but still think that they will come in handy during retirement. The big unknown is United's long term survival, and the survival of the MP program in the event they don't make it.
I've found using miles for an unplanned last minute trip is very hit or miss. I'd never count on using miles for an emergency trip.
It's interesting to see what other F/T's mileage hoarding strategies are - I'll re-evaluate my situation based on what some of you seasoned flyers have to say.