I've actually been pleasantly surpassed how frequently I use it—goofing off aside. I definitely use it for the "Call John Doe" feature, but I also use it for text messages on occasion (sometimes to initiate the message, sometimes to dictate it), text dictation, setting timers and reminders, playing music (including playlists or creating genius playlists), map lookups and directions, initiating searches, and a little more as I gradually figure out more about it.
Outside the Apple-specific features, I've found I'm using WolframAlpha queries more and more, and this use is gradually scaling with how much more familiar I'm becoming with WolframAlpha. It can actually do some pretty fantastic stuff. Hint: you can say, "Wolfram, [search]". for example, "Wolfram, what is the scientific name for weasel", or "Wolfram, random integer below 21".
Edit: the more I think of it, the more I realize I use Siri for. There's math, unit conversion, information gathering (e.g. asking who someone is, or getting the definition of a word), and more. WolframAlpha has some good examples which might be made to work.
I thought it might be good, but I didn't expect to use it for a whole lot. I never used the old iOS 4 voice control feature. I'm not sure if my fondness for Siri is because I've been using it more to become comfortable and familiar with it, or if it just has to do with my specific circumstances.