Personally, I find Siri useful for example dictation. Some tasks are quicker with Siri but others are not. The main issue with Siri in my opinion at the moment is the reliability ( less commonly used words and their context, I.e read,red / for, four - that is going to be difficult to solve ) and functionality outside of the USA.
I am looking forward to more integration in the iPhone for example to turn on / off Bluetooth etc. Integration with other applications would be great, and I don't think it would be that difficult - its just another interface - Apple can parse a voice command and invoke the appropriate API command.
BT headsets need to get better - the quality I find isn't good enough and with awkward integration - ie., having to press down a button for a couple seconds... I'm sure we'll see better integration soon ( I have a Jawbone ).
I have just dictated this entire message using Siri, it was far quicker than typing on an iPhone ( I have edited twice on the keyboard since :- ) )