I had crossfire 7850s for about a year, and it was a love-hate relationship. It was either a great experience or an absolute nightmare, all depending on the game.
With well supported games like Bioshock Infinite and Tomb raider it was fantastic, the scaling of adding a second card was >90%, the game ran very smooth, and no crashes.
However, when it was games with no crossfire support.....
My favorite game Chivalry Medieval Warfare was a nightmare with Xfire, it has zero support from both AMD and the Dev. With Xfire the game has frame rate issues, horrendous micro stutter, and each GPU was never utilized above 50% AND the frame rate was low (20s-40s).
The final straw was Titanfall, right after I saw the artifacting with Xfire I ordered a GTX 780 and have not looked back. It's been fixed since, but I don't want to encounter issues like this ever again.
Sure you can disable one card for games that are not supported, but that's bullshit. Having one card sit there being useless because the developer is too incompetent to support Xfire is infuriating.