Firstly, these ads appear in between Google Shopping results and the Gooooooogle pagination thing. Obviously that placement isn’t accidental: to get to the next page of results you are now forced to see the ads. That can only have a positive impact on CTRs.
Secondly, anyone who has used product links or sitelinks in their AdWords know what a dramatic effect they can have on CTRs too (I personally saw a fivefold increase in CTR for ads with sitelinks on a big enough sample size to know it was no fluke). What I haven’t seen – and I could be behind the times here – is the promotion of product links with no ‘plus’ button: the images are just planted straight into the ad.
It’s a very attractive format for an advertiser and one that would certainly appeal to me – especially if it were rolled out into the main SERPs.
Does that take Google another inch to being primarily a shopping and advertising channel to the further subtle detriment of the organic SERPs? Well, duh. Everything Google does at the moment is informed by that rationale.