I got my first (and only) Legion legendary a couple of weeks ago: [Ilterendi, Crown Jewel of Silvermoon]. So I immediately respecced to Judgement of Light.
It actually works pretty decently. Judgement into Light of Dawn into Holy Shock does a fair amount of healing. Judging more or less on cooldown contributes about 25k dps, and it doesn't seem to really drop my total healing.
It got me thinking about the general design of legendaries in Legion. In many respects, I think my experience is how Blizzard intended legendaries to be used. You get one, and you build your character around it. So each legendary gives you a slightly different playstyle.
But one issue is that you may not like that playstyle. I like Judgement of Light, but it is mechanically different from the other builds, and I can see some healers disliking having to keep an eye on the enemies. Not to mention that it generally works best with some macros to smooth things out.
Another issue is that some playstyles will strictly math out better than others. Ilterendi is consider the second-best Holy Paladin legendary, and pretty close to the best. So it's not an issue for me. But someone who gets one of the "worse" legendaries will be unhappy.
But if everyone gets to choose their legendary, or the drop rate is high enough that you eventually get them all, then everyone will pick the one considered "Best-in-slot". And that seems to negate the whole "build-around" aspect.
The other part is that the legendaries have secondary stats on them, and they might be the "wrong" stats for your class. I do think it would have been better if the legendaries only had primary stats.
Another possibility might have been to have the legendaries be more common, but they have a lower item level base. Then there could be Warforged or Titanforged versions. This way, it would be fairly easy to get access to the "build-around" component, but a player might choose to build around a Titanforged version of a weaker legendary.
Or maybe add an expensive device that allows you to transmute legendaries. A Kanai's Cube. That way if you really did not like the legendary you got, you could spend resources on getting the one you desired.
Also making it completely random was probably not the best of ideas. I think getting a random legendary from a quest to kill the last raid boss (in any difficulty) would have been a better way of handing them out.
All in all, legendaries in Legion remind me of garrisons in Warlords. The system is not quite there, but is pretty close. A few more iterations might have made it much better.