Let's put it another way. 16x QSAA would give you the same level of edge anti-aliasing as 16x MSAA, at the same cost as 8x MSAA. The trade-off is that textures will become blurrier, making 16x AF look like 4x AF.
MSAA is the quality option, QSAA is the speed option. In things like racing games, where you are moving too quickly for slight texture blurring to matter, but aliasing is super-apparent, then QSAA is a great choice. In something like Borderlands, where the unique texturing is a huge feature of the game, then QSAA is a crappy choice.
Personally, I say throw on 4x MSAA and 4x non-optimized AF and call it a day. Anything more is really a case of severely diminishing returns. Anything less is failing to take advantage of features that are essentially free on any moderately current card.