They've done a real great job with the artifact questlines. I did the Balance, Feral and Guardian questlines so far, and especially the Guardian one was gorgeous. Simply gorgeous. It brought us back to Northrend, expanded on Druid lore, gave us some backstory to the upcoming Emerald Dream raid and was in and of itself a wonderful scenario. I especially liked that it tested you as a tank, which is exactly how it should be. It reminded me of Wrathion's challenge in MoP. I'm honestly amazed how wonderful that questline was. It even made me want to raid as a tank more than anything else ever has, and made me want to go into the Emerald Dream and free Ursoc. And there is so much more of this. I'm someone who has always played all classes, I have a max level character of each class since Wrath. And I will definitely go through all of the questlines. And I think it will be a blast.
I got the Collector's Edition, and the 80 bucks are well worth it, the content of the game itself as well as the CE stuff. If I compare this to other singleplayer games, WoW: Legion has a great content to money ratio.
What bothers me though is, that the world feels so empty. I don't get why. I've barely met anyone in the last two days. And I'm on a full server. Only time I see a lot of people is on my way into the Dreamgrove, because that's crossrealm.
And even though the questing experience will be great, with how server communities don't even exist anymore, I feel that at max level, the game will not have a lot to offer. The first raid with only 7 bosses will only open up in a month, and then will release LFR parts slowly over time. There will be little to do and little reason to stay subscribed. That is when the content to money ratio shifts to the other side. Paying 45-80 bucks for all the content we get initially with the launch, all the questing content, that's definitely worth it. Paying 13 bucks a month for little to no endgame, at best grinding the same things again and again and then waiting for a rehashed Karazhan or possibly some other minor content patch - that doesn't really sound like it will be worth it at all.
So I recommend that everyone play WoW: Legion as the singleplayer game that it wants to be. It's great at that. Play through all the artifact questlines, play through all the order hall campaigns, play through all the quests in the zones and try out all the different specs. Playing it as an MMO, though? Not so much. Blizzard has moved away from making MMOs and moved towards crafting great singleplayer experiences.