If I used it in public I would probably get my asskicked/laughed at depending on the location. The only places I would use this is at home or in the car. Besides, I can picture a situation where I would want to send an inappropriate text to Yenissa (my girlfriend) and it got sent to Jenissa (my boss)...
I think SIRI will be useful based on the person. If you use it at home, an office, or in your car often then SIRI could be extremely useful. But, if you're in public a lot then the feature won't be very useful. It really depends on the person and their lifestyle.
Personally, being able to read/write text and emails in the car is huge. And, if the voice recognition works smoothly you can bet every other smartphone will try and copy it.