To some extent ... as you know, the winner's curse infects FA. That and the age of the average UFA makes it a difficult pool to jump into for longer term help.
If you look at some of the biggies
Sandoval - a shaky contract when it was awarded, the player has more than lived down to the pessimism
Ramirez - a pretty good contract considering. He was lousy the first year, and bounced back last year. Has been a mixed bag, but I'd happily do it again largely. (although I'd have put him in the infield from the jump)
Victorino - an MVP-caliber one season, which paid for the entire contract more or less. Probably a net loss but not a large one and flags fly forever.
Uehara - wish he were more durable, but 2013 was magic, and when he was relatively healthy he was always good.
AJ Pierzynski - he was awful. But clearly part of the intent was to have a 1-year stopgap because of how much catching they had in the org. So no harm.
There were lots of relievers otherwise, and they do what relievers do ... kind of throw your hands up there