I had problems with it loading other iframes on the page into the adslot running async. I ran it for a few days to test it but just pulled it because of this. The other iframe would overlay the ad iframe.
I emailed our Google rep about it months ago and never heard back on a fix so reinstated it a few days ago and it was still doing it. Doesn't look like they are doing much about it.
No, I used the async code and it PULLED another iframe into it from the site. The async code DOES create an iframe on the page, that's how it works. however, for whatever reason it has issues with other iframes on the same page.