When I need 12x12 layouts printed I go to Costco (they have 12x18 prints here and I just add 3 4x6 photos to the blank part of the page). If your resolution is high enough, it comes out great! My first attempt was poor because the resolution wasn't hight enough. I've learned my lesson.
I also print as 8x8 or 8.5x11 at home. The quality isn't as good as Costco though.