It depends on a game per game basis. Most games only give you some extras (such as in Mario Kart 8, you only get costumes for your Mii). A few games actually "interact" with your Amiibo, such as Smash Bros. I think training Amiibo is quite fun.. until they get really good
When I first started collecting Amiibo a few months back, I had the mentality that I would buy them because they look cool and/or I want some sort of hobby.
4 months later, I've barely collected around 25
People have told me that I started TOO late. I disagree. Some Amiibo went up to $100 in 2015. Now? Those Amiibo cost around $30. It's never too late to start collecting.
Anyways, it really depends on your interests. If you plan on buying them for games (especially for games besides Smash Bros), you might be a little disappointed, but if you buy them as a hobby or because how cool they look, I think you'll be happy.