There are a lot of posts on this subject. For the most part, it depends on where you're going. With $200-ish transcon fares these days, it makes little sense to use miles for such an award.

The best value for awards tend to be international, premium cabin flights. However, I've been known to use a 25K domestic from time to time, but only to expensive, odd-ball cities.

Of course, other factors play into it as well. For example, if this is going to be your last flight of the year and you're only a few miles short of elite status for next year, it might be worthwhile to spend the cash.