It can be good to keep a modest stash (150K for example). Once I tried to sneak in some diving on Jeju Island in South Korea at the end of a business trip, where I was to return from S. Korea to France to the US (complicated story, but because of work reasons to be first in France and then in S. Korea). A storm blew in as I landed on Jeju Island, and there was to be no diving for several days. Because I had a stash of miles I was able to call that morning and use miles to get home that same day (ICN to LAX to JFK, my home then) rather than wait 3-4 days. Would have cost me thousands of dollars if I had done this without miles.

I would say use the miles for new travel! It seems silly to spend them on upgrades! I would just use them for some fun travel!

The best "value" of miles is often premium travel (first or business class) on tickets that would normally cost thousands of dollars. There's always a debate then on would you actually buy these tickets anyway and if not, then can you really value them in terms of these premium travels.

For 400,000 you can book two Round the world (RTW) tickets...for 600,000 you can book two RTW tickets in business class. Would be a fun vacation for two if you're retired.

Otherwise, I try to use miles when I'm booking last minute and ticket prices are $$$$. Depending on flight and times, United for instance will often release open seats near departure to reward tickets.

Another option is United's Cash and Points reward ticket option. You get to spread your miles out farther and get to usually buy miles cheaper than 1 CPM.

As everyone said here, you never know when they will raise redemption values and if you've already made your desired elite status for the year, then using miles is a good way to keep flying and save your cash for earning status next year.

I flew last year to Turkey and London on Lufthansa first class and Turkish airlines and I have to admit eating Caviar in First, being personally chauffeured around the Frankfurt airport, and having the entire plan held up until I depart the plane was one of those once in a lifetime experiences. That trip cost 135,000 miles and I'd gladly do it again.

Keeping a stash of at least 150,000 is a great idea as well!

A common position and perhaps even accurate in most cases but not always. Programs do become enhanced with more partners and routes and the fares do go up (or fluctuate). I'm guessing there are more routes today you can redeem with AA miles than 5 years ago.

My approach is to use systemwide upgrades on paid fares (AA) to requalify and use miles for business/first awards on partners. My balance usually is around 400,000 miles in the primary account and 150,000 on a couple other carriers/alliances.

I have my own set of rules on when to upgrade. As my travel is > 95% domestic, I don't have to worry about large copays.

Rules:
1. There must be more than one flight (preferably three)
2. At least one (preferably two) must be a meal flight
3. Hopefully, at least one is a widebody/has enchanced seating (widebodies, intl. style cabins)
4. No upgrading with miles routes that should easily clear with free upgrades.

However, if I had 300,000 miles I'd try and plan a RTW or something similar before I started thinking about upgrades.