OK, this is too cool!
My new iPhone 6 was in the bedroom, on its charger; I'm in the kitchen eating breakfast.
I got a call on the iPhone; my iPad(!) in the kitchen with me, "rang" saying I had an incoming call from <name>, and would I like to answer it?
I clicked yes and took the call via my iPad instead of running into the bedroom to answer it.
Postscript: Now I'm at my desktop computer. A notice had popped up there too, asking me if I wanted to answer the call via the desktop.
The reason I'm surprised is that from everything I've read, this feature requires newer model hardware than my iPad 2 and 2010 MBP, both of which lack the "required" BlueTooth-4 LE hardware. Apparently, it will work as long as all the devices are on the same Wi-Fi network!
I believe I saw in the ADC Keynote that the feature works both ways -- you're supposed to be able to initiate a cellular call from your computer/tablet.
Does anyone know how to do this?
I don't see an app for it.
Answered my own question about placing outgoing calls:
If the person you want to call is in your Contacts, you'll see a little telephone icon next to their phone number. Clicking it on the OSX computer or iPad will bring up a dialog offering to place your call using the iPhone (even if the phone is asleep).
But there doesn't seem to be a way to call a number not in your contact list?
Looks like I probably won't be able to use this feature as it does not work if you have WiFi calling enabled. I plan on keeping mine set to WiFi calling enabled.