I've been testing Naryu's Passion for the past two nights and I think that it has worthwhile uses and some potential. Love and Rejection are probably a tad better as of now because they're useful in more situations, but Passion is also a decent offensive and defensive option.
I've found it useful for punishing in some situations. The 16% damage reward is nice and is good to use when you're out of range for a Farore's Wind or a grab. You need to know the limits of its range when you use it though; if you use it incorrectly you can obviously get punished yourself.
It can also punish landings when the opponent is close enough. This can work particularly well if the opponents air dodges when landing. You do have to be careful against characters with fast aerials, but it can still work against them if used right.
Finally it has a use offstage for edgeguarding purposes, though it should be used very rarely if at all for the speedy aerials characters, as it's not hard to get hit out of Naryu's Passion in the air. This seems to work decent on characters like R.O.B and Donkey Kong from my experience (though you'll have to make a trade with Spinning Kong/Kong Cyclone with DK). It also should work on Diddy in theory, but I haven't pulled it off yet on him, so I'll have to test more with that.
I've only been able to test on lv. 9 CPUs so far, but I'll try to get someone to test customs with me on Anther's Ladder in the near future so that I can test on humans.
I use a set of 3211 in these matches. Dunno if it's a CPU-only type of thing, but Din's Flare and Phantom Slash were putting in a lot of work.
EDIT: Volume is pretty low on the videos