There seems to be issues that are not fully resolved. There's some sort of initial exchange required and once it's done for each, everything works. There are exceptions when it comes to Pages, Keynote and Numbers.
So to get the stuff working that is not working yet, lock the screen of your iOS device and then open the compatible app in Yosemite. You'll see the compatible app icon on the bottom left of your iOS lock screen. Swipe up and unlock your iOS device. Now close down the app in Yosemite and quit it on your iOS device. Re-open the same app on your iOS device and you should now see the app icon appear in Yosemite. I have found that if you follow these steps, Handoff will work every time, in both directions with all of the compatible apps except for Pages, Keynote and Numbers.
For Pages, Keynote and Numbers, there are additional requirements and exceptions. These aren't officially documented requirements that I found from Apple, but instead just what I found works (with limits) for now.
1. You have to have iCloud Drive active
2. You have to share the document on iCloud from your Mac (Goto Share>View Share Settings...) - Be patient, it's not fast.
3. You have to have "Use iCloud" enabled in the app settings on iOS
Document handoff from Page, Keynote and Numbers on Mac to iOS works every time if you first share the document, but sharing from iOS to Mac doesn't work unless you have first shared the document from Yosemiteand sometimes from iOS you also have to tap Share>Share Link via iCloud>Copy to get the icon to pop up on your Mac. Enabling the share setting for Handoff from iOS is not possible. The share settings only appear for files that have been first shared from Yosemite.