Have Apple added much to Siri since it launched? Certainly, I don't remember any big banner features.
Now, ¿Does that mean that Siri hasn't evolved at all? Not necessarily. Server-side it might have become more reliable or accurate without Apple promoting it, for example.
Personally I use Siri as a shortcut for some functions that otherwise will require to many taps or navigation. Every day I set up my next day alarm with it "Wake me up at..." and sometimes I set up appointments using it. I might ask her to read me some message if I'm running, too. Some of those things Google Now / Android does not offer in it's current state.
I've never had any reliability problems so for me the service has not improved. Maybe users with thicker accents or that had trouble at the beginning would be able to tell otherwise. This has been the case, for me, with iOS Maps. Not new big banner features since introduction but constant improvements without publicity.
On a side note, another two cents: Siri's buzz didn't have to do with the product itself but with the glimpses of the future it offered. It carried the message that sooner or later our way of interacting with things will be a mix of gestures and voice.
It faced the same problems other speech recognition systems and virtual agents had in the past (including pre-Apple Siri) but the fact that a company that caters to the mass market was ready to give it a push probably caused a significant shift and raised awareness to this technology. Certainly has become a well known feature even by common, non-techie users. My mom or my grandmother knows about it, even if they don't use it. That is a huge step forward, in my opinion. Theres always time to refine and improve.
The fact that Apple doesn't have a web search engine boggles my mind, they only need look at how Google and Microsoft exploit the Web corpus to create really compelling user experiences to know that its rapidly going to become the differentiating factor between integrated product offerings.
So there is one area that Siri definitely beats Google Now and that is usability from day one. It is an important criteria for a mobile app.
I do not speak English in an American, British or an Australian accent and I disabled Siri on my iPhone in less than a week after I bought it.
This alone expands Google Now's market by a significant number.
It reminds me of this really lame ongoing narrative that the tech press had 10 years ago, about Bluetooth vs. WiFi and which technology was going to, uh, "win". Aside from being based on pushing bits through RF, they were completely disjoint technologies that solved different problems, but a bunch of tech pundits kept writing articles about how there was some battle for the future of wireless communications and one of them was going to destroy the other.
I'm still wondering how the hell anyone could seriously consider releasing this abomination.
Until Google decides to join the iOS infrastructure (like reading from your Contacts - radical I know) none of it's apps will provide even the slightest interesting interactions.
start voice listening on both (one Siri, one Google app) at the same time
see the difference
EDIT: Thanks for the video. Summary, Google's voice search is the same speed or faster than Siri for all queries tried with the results being nearly identical in all cases.
EDIT 2: Here is an updated video from the same person six months after yours, but still six months ago, wherein the difference is less clear if not in Siri's favor.