Yep, I've played, completed and loved them all. Plus I live in Europe, and therefore have FFX: International. The Dark Aeons are extremely hard unless you have stats in the mid-200's (max them out for Penance), and HP of about 30,000 (Except Dark Valefore - his attacks only do a max of 9999 damage, they don't break the limit.) I haven't reached Penance yet, as you need to kill all the Dark Aeons to access him (you need to fight Dark Yojimbo five times in a row!) but he's ultimately the baddest geezer in the game. Even stronger than Nemesis.
The downside to them? Well, you cannot return to Besaid, Macalania Temple or Zanarkand Ruins (the room where you fight Yunalesca) because as soon as you aproach them you'll be attacked by Dark Valefore, Dark Shiva and Dark Bahamut. So to return to any of the temples to retrieve forgotten Destruction Sphere items or to gain the secret items in each chamber of the fayth you'll need to beat those Dark Aeons. Dark Ifrit is way out at the ruined Home (Sanubia Sands), while Dark Ixion is under a lightning rod on the Thunder Plains. But those two can be easily avoided if you aren't prepared for battle. Dark Yojimbo hangs out in the Cavern of the Stolen Fayth, but he's easy to dodge as you need to go to the back of the cavern and walk back through to initiate battle. Dark Anima is the hardest to find - you need to complete the Gagazet Trial (The Blitzball throwing) again and then she appears at the Gagazet mountain gate. Dark Magus Sisters hang around the start of Mushroom Rock Road. Their summoners surround you and if you touch one the battle will start, so you need to do a bit of ducking and dodging to pass them safely.
Phew, rambled for a bit there :D. But yeah, huge Final Fantasy fan here. Yes.