I guess you are referring to the "full-time manual focus" feature which allows you to do manual focus without switching from AF to MF mode.
I find it very convenient if you combine it with back-button autofocus. Then you will never need to fiddle with the AF/MF switch. You can just leave your lens on AF all the time.
If you want autofocus, it is immediately available in the back button (just like with any autofocus lens). If you want manual focus, you can directly turn the focusing ring (as if your lens was in the MF mode).
And you can combine these any way you want. For instance, you can first AF and then immediately after that fine-tune. Or if you notice that AF starts hunting, you can simply give up with AF and use manual focus.
Of course there is a downside, too: it is easier to accidentally change the focus when you are handling the camera.