DO NOT LEAVE IT ENABLED!
It's not all sweetness and light as the name suggests, I'm afraid. While the option does allow you to load save games from other people, with no ill effects for that game, it has the unfortunate side-effect of faking the owner of EVERY save you load - even the ones you already own. I've had FSO enabled since I installed Rebug 3.55.1 back in April, since I sometimes like to test downloaded saves, and I've not noticed anything untoward.... until today I was randomly checking my saves on the machine, cleaning out older ones from games I no longer own or play, and I noticed a few of them had their owner status removed - so I used "Check Owner" and saw "NotOwner(Online)" on a dozen or so games, but "Owner(Online)" for others. I quickly switched back to DEX Menu 2 and disabled FSO, then loaded a game where its save was NotOwner.
.... the game complains that the save is corrupt, or doesn't belong to the current user
Gran Turismo 5, Dungeon Hunter, Alien Breed, DeathSpank, FFXIII, Valkryie Chronicles, Dead Nation, Dead Space 2, Mortal Kombat. All gone/corrupted. Basically every game I've loaded/saved in the last two months since installing 3.55.1 and enabling FSO. The only way they'll load is if I re-enable FSO, which I'm definitely not doing again - lest it corrupts every other game I own. Thank god I haven't loaded Heavy Rain or Demon's Souls in the last two months
Let this be a warning to you guys! Use it if you absolutely have to, but make sure you DISABLE it when you're finished playing a game that is using somebody else's save file. 'Kay? Trust me on this. I've lost a lot of hours in a lot of games