I tried it because I watched an Adsense video ad that almost convinced me to buy, and would have if I didn't already have a fairly new gizmo that does the job, just not as cool and snazzy as the one in the video.
OK, I just checked it out, and it isn't the graphic ad. It's happening on pages with just text ads on the top and bottom both (which imho can work well on some pages), which wasn't happening before that I recall. So if they're giving advice in their optimization tips to put more ad units on, they are WRONG until they fix it so that the preview tool is usable.
This is a real dilemma, because some pages really do need one on top and one on bottom, and I can tell because products displayed on the page have sold/converted from both the first and the last products displayed, which means they're both good spots on those particular pages. Not all pages, but with those layouts it's a shame to miss out on opportunities.
Why in the world should a page redirect to the very bottom when the preview tool is opened, just as it would with an anchored link (which the pages don't have).
I just tested with just one leaderboard on top - it scoots (like an anchored link) to the very top line of the one single leaderboard on the page, whether you're hitting the preview tool from either the top or the bottom of the page.
So.. wherever you are on the page when there's an ad block on the top and the bottom both, it will scoot you to the very top line of the BOTTOM ad unit, so that at no time can you view the top ad unit and the preview tool at the same time.
It just doesn't work with more than one ad unit on the page, so it's virtually useless for checking out the ads on top without pencil and paper notes which is rather primitive to have to use for a high tech gig like Google.
Unless of course you have crackerjack short term memory for a lot of details and not middle-aged memory, which ain't me.
[edited by: Marcia at 5:53 am (utc) on May 31, 2007]