Y'see, given that it exists, I don't see why this isn't the single most frequently brewed potion of all time. Using felix to inspire you to work out how to do something as major as convincing someone to reveal one of their two most shameful secrets means that it is capable of producing some spectacular effects - possibly more spectacular than you can deliberately conceive. Arguably, if the best way to deal with an enemy is to turn him into a friend, using felix is more powerful magic than Avada Kedavra.
We do read that the potion is "desperately tricky to make and disastrous to get wrong", but evidently not such an imposition that a new potions master cannot produce it in his first lesson. The HP Lexicon doesn't confirm or deny, but I'm sure I read that the potion takes six months to prepare, and Wikipedia backs this up. It seems unlikely that Slughorn had been able to brew potions for the previous six months, given his travelling existence, so presumably the lesson's sample was some he had made earlier. I also wonder about the fact that a small cauldron of it is bubbling, but only a tiny bottle of it is produced as a prize. Either there's some pretty severe distillation between us seeing the cauldron and the full bottle - perfectly possible - or there's a whole load of felix going round unbottled somewhere.
Other than the (debatable?) difficulty of producing the potion, there is the argument that it is toxic in excess and the aspect of legality. I would have thought that any well-organised, properly-thought-out organisation intent on evil would stock up on felix in advance and break it out when required. Chapter one states (wonderfully!) that "the trouble is, the other side can do magic too, Prime Minister" - unless Slughorn is uniquely capable of brewing the potion, I don't see why the other side can't, or don't, use felix as well.
The way that JKR has Gred and Forge branching into the defence industry by creating and importing lines of Shield Hats and Instant Darkness Powder, only for the Instant Darkness Powder to be used against Dumbledore and his forces, is an excellent illustration of this principle - plus, if you care to interpret it this way, a cute dig at the real-world sale of arms to recent allies who later become enemies. There's no reason why, by extension, felix couldn't fall into the wrong hands too. Certainly Death Eaters wouldn't worry about issues of legality or the substance being banned and might quite possibly be prepared to risk the effects of overconsumption should short-term gain be critical.
The apologist's leap here would be to suppose that we haven't learned that felix borrows from the great magical tradition that when a wizard casts a very powerful spell, or creates a very strong magical item, then it is at great (permanent?) cost or injury to themselves. It's just conceivable that this could be the case but Slughorn didn't mention it at the time for some reason. Other than that, if there's a crucial mission (like killing Dumbledore) then I'm going to make darn sure that I've taken felix beforehand, just in case the people I encounter have done - as they had, in this case.
There's fanfic to be written where malign forces have achieved spectacular, improbable victories through use of felix...