Don't have HL2 yet, though 2xQ is growing on me rather quickly. I just tried it with BFV and I gotta say, I was happy to see near 4x quality while being much more smooth and responsive. Playing @ 1280x960 2xQ/4xAF. Text is blurry, however I can live with that. I use the keys and mouse to fly the heli's and planes, so it is vital to have smoothness in gameplay as well as the controls being responsive. Beats running the game without AA.
Running CoD: United Offensive at 16x12 4xAA/16xAF and smooth/responsive.
I'll be sure to give this a go when I do pickup HL2 though.
Didn't notice any objects being manipulated by 4xAA in BFV like they can be with HL2, well.. not as obvious at least.