I'd be wary of touching any Yahoo API that isn't a core part of their business. Developer enthusiasm isn't enough considering how likely, when Yahoo run into difficulties, they'll just sunset APIs we're using.
I want to say the Search API is a good example of a core business proposition: there's a revenue stream there for the service, but the search results are probably from Bing, so although the Yahoo side looks like it has longevity, the reliance and licensing of search metadata from Microsoft is a dependency that ups the risk of using this API. I don't really trust Microsoft not to pull the carpet out from under us when they've collected a sufficient number of eyeballs to consider the second step of a bait and switch.