The short answer is user research.
Regardless of the feature, application, platform, or specific task at hand, questions around prioritization of R&D/feature addition should always start with user research.
In your specific case, even if 75% of all OS X + iOS users utilize Keychain and 45% use Handoff, your specific market may over- or under-represent in usage of either feature. Thus, the answer for my product development question is different from the answer for yours.
However, user research is a well discussed topic. David Sherwin of Frog wrote a good article laying out their five step user research process and A List Apart has a great article from Rachel Andrew, Managing Feature Requests.
The biggest takeaway from Andrew's article is this:
Realize you are not your customer
Like so many other products that have been launched by consultancies, we built Perch to scratch our own itch. Version 1 was very much the product we needed: a product for people who cared about web standards and structured content. We then had to be willing to learn from feedback. We had to accept that some of the things we thought we should decline were real requirements for the people we felt were an ideal fit for the product.
I highly recommend taking some time and doing a deep dive on conducting good user research, and then craft a few solid questions to ask your current or intended (or potential) users. You could probably start with the question you asked above and get some really good insight.
For further reading, there are quite literally thousands of pieces on the topic, though I would personally direct you to the the User Research tag on UXmatters after reading the two short ones above. And of course I'd be remiss if I didn't also point to our very own, fairly well answered user-research tag.
And finally, sometimes it's perfectly fine to build something cool just to make a handful of people smile at the thought and attention put into the functionality of a tool.