Google's SVP of Mobile Andy Rubin spearheaded an attack on one of Apple iPhone 4S' most important features - voice assistant Siri - saying the phone is a tool for communication and should not be an assistant.

You shouldn't be communicating with the phone; you should be communicating with somebody on the other side of the phone, Rubin said on Wednesday at the AllThingsD AsiaD conference.

But Rubin admitted that it is natural for people to talk to their phone to some degree. When referring to speaking to the phone rather than trying to talk to another person, he said he is not so sure. We'll see how pervasive it gets, he said.

Meanwhile, Windows Phone Chief Andy Lees also commented that Siri is not super useful. He said the similar voice assistant tool found on Windows Phone 7 Mango is more useful than Siri, according to Engadget.

However, Lees' superior, Microsoft CEO Steve Ballmer, was more kind. Ballmer said Apple is a good competitor, but a different one.  Both [an iPhone and a Windows phone] are going to feel very good in your hand and both going to look very beautiful physically.

After Rubin downplayed Siri, one article from Business Center criticized his opinion, saying Rubin either doesn't understand Siri, or he does--but he'd rather not admit it publicly.

Meanwhile, ZDNet on Thursday listed 5 reasons that Siri frustrates the author, including, Siri doesn't observe the silent rule, Siri still makes a lot of mistakes, and Siri has no offline mode.  But the author also said he has been enjoying Siri, and completely surrender to Siri for some of my more important tasks.