I'm sad at this. If they wanted to swtich, I'd have prefered Bing at the least. In term of quality search, as far as I'm concerned it goes Google->Bing->Yahoo->DuckDuckGo
As an example, I was doing a search to see what are my options to check Boardgamegeek on a mobile without using the main website as it's not mobile friendly at all. So I did this search on all 4 engines: "boardgamegeek on mobile"
Google and Bing showed as first result the bgg wiki page on mobile access which has all the relevant information. Yahoo, even though it's using Bing tech, never showed that search result anywhere in the first page, neither did DuckDuckGo, and their search results didn't gave me any answer I was looking for. So, useless in the case of this search.
So, pretty sad that they chose Yahoo :(