I like it when it works, and I think the # 1 thing that causes it to fail is your internet connection. The ideal time for me to use it is when driving to text or, rarely, call someone. Unfortunately, you're in motion and your data connection may be compromised at any second. There's actually a stretch of road that I take where Siri fails every time even though I have no issues with phone calls on that road.
What would be nice is to have the software be more independent, like with calls, texting, alarms, lists, scheduling, ect. Obviously, if you want to do a google search and other net related stuff you need a web connection.