Y! Mail becomes more usable with the extension installed, but I prefer using a different search engine, in particular, I like Google's specialized searches, like the one you get when you search for a song.
There is a greater concern, however. I have had trouble with security on a company managed machine and lots of reports for unapproved activity. Mostly when I use Yahoo Mail. I have so many when the extension is enabled, that my account is locked.
I had no time to investigate more deeply which kind of requests come from the extension, but as a precaution I have uninstalled it completely.