It’s interesting data and I appreciate Google providing it since it gives me a better idea of what sort of click thru rates you can expect for a certain keyword position. My problem with the impressions data is that it doesn’t match up with the data from their own keyword tool.
For example, one keyword that we are consistently ranked #3 or #4 shows in Webmaster tools as providing 1,900 impressions over the last 30 days, but on the keyword tool it shows 9,900 searches globally and 5,400 searches locally in March. The timeline doesn’t quite match up since Webmaster Tools doesn’t appear to go all the way back to March 1 right now, but still the data is quite a bit different, so it makes you wonder which is correct. My guess is that the lower number is correct which is a bummer since I use the keyword tool data quite a bit.
Dom, if you click on the plus sign next to a keyword (on the page showing the graph) you get data on your position in the search results and related impressions, click thrus and click thru percentage. This part of the data rocks actually.