Siri is pretty good, but in my opinion there are few cases where it's really that much faster than using the touch gestures. It takes a surprising long time for Siri to respond since it has to contact Apple servers for every request. For example, I can set a 3-min tea timer using the touch UI faster than asking Siri to do it for me.

Also keep in mind that Siri is terrible in any kind of noisy environment, especially with other people talking.

I will say that Siri excels at playing specific songs, artists, albums, etc. It's much faster than using the touch UI. Siri is also good for when you don't have your hands free.

Overall, Siri is pretty amazing but it hasn't fundamentally changed the way I use my iPhone 4s. I will say the iPhone 4s is ridiculously fast and smooth, although I upgraded from an iPhone 3G so YMMV.