I am also seeing incredibly wild/random swings in GWT Search Queries and have no idea what to make of it... in part I don't know how to interpret since GWT Search Query only went live a month or so before the MayDay debacle, so I don't know if occasional wild fluctuations are part of it.
What I see is a steady drop in the baseline impressions and CTR over the past 5 weeks, with sharper drop on May 31. There are occasional wild oscillations where the Impressions counts goes up by 8x - from roughly 4k/day to 30k/day for one-two days, then down to 3-4k/day. On days where Impressions skyrocket, the Click count typically drops, though, ie CTR goes way down.
Prior to May, Impressions were roughly 8-9k/day and this was very steady, never varied by more than about 15% (from memory).
I can also confirm that the pages from my site alone which now rank are far less relevant, and the pages in the top 10 are from nearly-identical affiliate template sites which carry identical inventory. These sites build tons of links to parent pages, not individual deep pages. There's a severe usability drop in those pages since users must scan them to find the proper relevant page and click a few times to reach it.