I'm an Apple user, so I'm most familiar with Siri. I've also used Google Now on a PC and iOS. With the advent of the technical preview of Windows 10, I finally had a chance to play with Cortana. (Just be aware that I'm biased toward the Apple platform and this may color my comments.)
So, how do they work?
They all have voice recognition interfaces and they all seem to hear and understand most of what I'm saying. I think Google Now may be the best in terms of accuracy. As far as the usefulness of the responses, Siri lags. Instead of giving me an audible answer to a simple question, she tends to "find" things on the web and then show me the web page. Siri is pretty good at doing things like making phone calls and setting reminders and tasks – that's how I use it most often. Google seems to do the best at reading me answers to my questions, and does pretty well with follow-up questions. Cortana mostly provides its answers on-screen.
While Siri is good at setting reminders, Cortana has a cool reminder feature – you can tie reminders to people. So if you say "remind me to say happy birthday to Jan next time I talk to her", Cortana will remind you if you get a text or email from her or if you have a meeting scheduled with her. (I should say that I have not actually used this feature as I don't do calendaring on Cortana.)
Ease of use
I think Google Now is the best – you can just say the "OK Google" keyword and it will hear you (there's a setting that you need to make in "Settings" to turn it on.) With Cortana and Siri you have to search or long press the Home button. You can have Siri listen for you to speak, but only when your phone is plugged in and charging(?) Just being able to talk to your devices a la Now is really the most convenient. This works on the iPhone version of Google Now, but the app has to be running in the foreground. I've only used Cortana on a Windows 10 preview PC and the only way it works is to get to the search field – once you're there you can type a query or ask Cortana.
I think Google Now works best at integration with the rest of your life (particularly if you use gmail and other google tools.) And since google's business is centered on search, I think it typically provides the most thorough answers.
Since I'm primarily an Apple user, I've spent a lot of time using Apple software and am most familiar with it. I'm generally content with Siri, but honestly the more I use Google products, the more I like them and Now is no exception. Cortana is still too new and other problems with Windows 10 stability affects my perceptions of it (I'm using a really early preview and I expect this will improve dramatically…) If I were using a Google phone, I would likely choose Google Now. For desktop use, I think Google Now works best.
Are you using any of these? What are your impressions?