There is an inherent friction in using any Software or a Gadget. A piece of technology or service that removes/reduces this friction is going to win BIG!
Some successful examples:
1. DropBox
2. Swipely
3. Kayak
4. Instapaper
5. GPS Voice navigation
6. Multi-touch interfaces
....
I believe Siri belongs to same the class of technologies..
HCI has still not reached 1.0