I have been tempted to buy a few recently, but I'm not sure how much they actually add. And the fact data can only be saved for one game at a time is a bit disappointing.

To the people that have some, are they worth it for what they add to games?

They are official Nintendo toys that allow you to get new (although tiny [skins, weapons, etc. for the most part]) content for your 3DS and Wii U units. That's the value of these figures.

I'd say you should get those ones that have compatibility in more games, such as Link, Mario and Kirby.

Well, my opinion is that they actually are quite fun to play with in Smash. At first, I was 100% in your boat: I was sort of interested but not really sure how much fun they'd really be. I bought a few and after playing with them in Smash for a while I really really enjoyed it. I have 9 now and I love playing with all of them. It's just really cool seeing how you can teach them things. They really do learn from fighting you. Also, I wouldn't really worry about the only being able to save to one game thing. So far, the only game you can save to is Smash. In the future, there will likely be other games you can save to, but they will have their own line of Amiibos anyway. Remember, this is the "Smash Bros." line of Amiibos.

Most are but not all. Some like Marth look disgusting

What other reason is there to buy them besides collecting?

Currently it's one game that saves data on the Amiibo: Smash. That gives you a more challenging computer enemy and an enemy that grows more with your abilities. Also you can take it to your friends house. Besides that Amiibos only unlock things or give you ingame-stuff. The decision is yours if that justifies a purchase. If you don't collect I agree with Metallox advice to buy the amiibos that are compatible with more games. So far that means Mario Kart, as Hyrule Warriors supports all Amiibos (although Link and Zelda are special) and Smash supports all that are produced so far. I don't know about Toad.

I didn't buy it and got it as a gift from a friend, but so far I don't really think they are that useful. I mean, I hardly use it, but I do like keeping it as a toy and I'm thinking of buying another one.

Yeah, I will be interested to see what different Amiibos do in different games in the future.  I think the fact they are so limited in terms of the games you can use them on right now is holding me back

Yep. I think they will release all fighters from Smash first as Amiibos, before they switch their focus to the next game. But still the unlocking (like in Mario Kart) or the presents (like in Hyrule Warriors) are nice. I'm sure many of the upcoming games will support stuff like that.

And about only one safe I can also say: use one or two for Smash and the others for other games.

At this point, I really only plan on collecting the figures for Donkey Kong and Zelda franchises.

I really hope there's eventually more depth than just skins, weapons, and bots. I want these Amiibos to really lead on to whole new adventures. I think that will happen eventually, but I'm just not sure when.

I'm praying for Wario-themed single player missions in Devils Third that unlock with the Amiibo