I love them.
To me they're fun collectibles, fairly cheap and the rarity makes a little bit more fun (I'm not going out of my way to find them, and refuse to do business with scalpers). When I go shopping I always pop into the shops that stock them to see if there's any I want.
The content that they unlock is almost negligible, and Splatoon will be the first time I've got Amiibo for content (and also for the cute figures), in fact, in the past I've got Amiibo and then waited months to register them. I think the content locked is (generally) pitched at the right level, striking that balance between 'physical DLC' that people keep bleating on about, and being so inconsequential that the Amiibo concept is pointless. A new costume in MK8? Sure, why not? But I wouldn't pay money for that anyway.
Amiibo Tap, IMO, is one of my favourite applications of them; tap any amiibo and get a random section of classic Nintendo game. It's fun, it creates value, and it's free. Nintendo needs to do more free 'apps' that utilise Amiibo, a collection app that tracks what you've got, has facts about the character (I'm thinking similar to the trophy hoard in Smash) so on and so forth would be a good one (really, it should have been incorporated into Tap).
In essence, I see Amiibo as cute collectibles, that add lots of little bits of value to my gaming. The Splatoon Amiibo are the first I've bought that are only compatible with one game (plus Tap), but I am buying now into the premise that they will be used elsewhere.