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.