Get an ICON.
I have an iPhone 4S. I had an ICON, which I accidentally killed. It worked great and easily worked with Siri. Then I got an Era. It doesn't work with Siri. Read between the lines above. IF you turn off some Era features, IF you install a app on the headset (which, contrary to Jawbone's assertion, does NOT improve the Era's functionality). IF you have to take steps that IF taken will marginally improve your performance, then it doesn't work.
So, I returned the Era, got a BlueAnt. It was better than the Era, but doesn't directly access the iPhone - you have to instruct it to access the phone verbally, it's stupid. Also, it doesn't automatically reconnect after being disconnected, which is an additional pain in the rear.
Today I went out and bought another ICON. And as before, IT JUST WORKS. No extra commands, no special apps, no need to deactivate features. Long press the talk button, and boom, Siri. Neither the Era nor the BlueAnt do that.
Nobody ever complained about the ICON when I last had one, and the sound quality is not remotely worth the ridiculous non-functionality with Siri.
The short answer, though, is "no" the Era does not work well with Siri or the 4S.