This is of course a "non-answer", but it really depends on your own travel patterns.

If you aren't planning on saving those miles for some TPAC or TATL F award, and you fly mostly domestic, and don't want to sit in Economy, and are out of (or don't want to chance using) e500's, sure, 15,000 miles to upgrade is fine.

As others have pointed out, they do give you a longer lay-over time than other upgrade instruments. And until the co-pays policy starts, they aren't necessarily a bad use of miles.

Maybe you live somewhere, as I do, where it isn't easy to get award tickets with your miles, when it is convenient for you to travel. If one isn't a 1K, with access to CR-1's, etc., your choice is to gamble on e500s (and maybe the House nearly always beats you on this one), or use the miles and gain some peace of mind with a confirmed upgrade. Certainly, in that case, GO FOR IT. Is domestic F worse than international F? Yes, but if you are flying mostly domestic, that question doesn't apply very often. Is domestic F nicer than domestic economy? Yes, and if you are like a lot of us, with mostly domestic travel patterns, why not enjoy the at least slightly better seat and service in domestic F than you will get sweating it out in E+? \

Work on enjoying the life that you live now.