I actually joined on the same day as Marissa so I can't really comment on how the vibe was beforehand. That said, I do approve of the job that she's doing.
The difference (to me) is pretty dramatic, and the confidence of being able to get stuff done is night & day. It still boggles my mind to think about how much has changed in just 1 month since she started. Fun times at Yahoo, that's for sure.