I've been thinking a lot about 'ad blindness' the past few months and if it really even applies to SEM.
Ad Blindness was originally used on the web as 'banner blindness'. Meaning, if someone put up 10 million ads in a single month, after a while - you just ignore the banner as it was just part of the background.
Most of the early banners were untargeted. Often they were ROS, and one could easily see the same banner 100 times in a day.
These banners weren't related to what someone was looking for, they were just banners. This is the major difference between banner advertising and the 'ad blindness' concept compared to search.
Search is based on someone looking for your product. They are proactively searching for something (they might not know what yet - that's why they are searching). Therefore, text/image ads that are relevant to what they are searching for have a higher chance of catching someone's eye.
Once someone has found what they are looking for, do they really search for your product a month later looking to buy it again? For most products, it's a simple no. They already own it, and the people who are seeing your ads are a new set of searchers. With new searchers, ad blindness isn't a relevant concept.
For those who bought your product once, and looking to find you again - this is why one runs 'branding' ads (i.e. bid on your company/product name) - to make sure those repeat customers can find your site.
Of course, there are many buying cycles that involve many searches, and various lengths of times. This is the only place where banner blindness could have an effect. Although, if it's a blindness or a branding effect as they're not going to see it thousands of times before they choose a product, is really the question.
I've run a couple campaigns where the ads haven't changed in over 2 years. The CTRs have remained a steady 10%+ - and these types of campaigns just seem to show that there is no ad blindness in search marketing.
It's an interesting concept, and I can see arguments for it either way. However, I'm more inclined to think that for the vast majority of products, ad blindness doesn't apply to search marketing.