Here was a recent thread/argument about it
https://forums.ootpdevelopments.com/...d.php?t=306436
If you mean the real life versions of it. Unequivocally, it is a useful, but imperfect, tool for valuing players.
If you mean the OOTP version, that's less clear. I haven't looked at it the same way I have real WAR (running a correlation between team wins and WAR wins), but I have some doubts about its accuracy. Especially with pitchers (FIP seems especially wonky) and, maybe, replacement level.