At the end of the day it's all about your own personal opinion and what you value your own miles at. I've done it both ways. Normally if the price is right I'll just pay for economy for domestic flights but sometimes it can be worth it to splurge and use miles for F. I personally prefer to use them on longer international flights but it's up to YOU.

You'll probably get 50 different responses all from people who have different mileage balances, upgrade options, and opinions.