This Vnu.net news article reports an interesting opinion of Yahoo representative:

"Poor usability is the main reason behind the limited adoption of Google's services such as Gmail and Google Talk, according to Jeff Bonforte, senior director of real-time communications at Yahoo."

Link to the article including more opinions is
http://www.vnunet.com/vnunet/news/2184316/google-usability-lacking

Watch the video:
http://www.siliconvalleysleuth.com/2007/02/yahoo_to_google.html

The author of the article exaggerates a biased opinion. The guy from Yahoo mostly talked about Yahoo Messenger and how their users think it's cool to share photos.