My daughter and I both have iPad Airs. She loves hers and never has a problem with it. Mine is great except that Safari almost always has to reload internet pages; makes it slower to cruise the net than I would like. My dd uses hers differently than I do. She is often on Facebook and Youtube for long periods of time. I am often trying to read forums and news channels on mine. We have the same model of iPad but purchased several months apart.
Recently I bought a Samsung Galaxy Tab S for a friend. We got him the 10" screen to make it easy to watch movies (plus I just like a larger screen). It also comes in a 8.5" size that is very portable if you like to take a tablet with you. I must say the screen display on the Tab S is fabulous, plus it has a micro SD slot so you can store an incredible amount of photos and videos on it. (The iPad, in contrast, does not have a micro SD slot. Once you run out of memory on the iPad it's a good idea to buy Cloud storage for your extras.)
The big advantage of iPad for us was that it was an easy introduction to the world of tablets. It's just easier to get started and operational than android. Now that we have some experience, android is becoming more attractive -- especially the Samsung products. The Tab S would be the one to compare to the current iPad Air II. If you Google, you can find several comparisons of the two products. Not everyone needs top-of-the-line in these two brands, though.
My best guess is that either of these will last someone a good 10 years before needing replacement (primarily due to changes in the operating system; once an OS becomes too old, many of the newer apps won't work well with it).
A decision really depends on how you want to use the tablet. I see a lot of positive comments about the Kindle Fire, but that may be due to how it is used. I have read quite a few comments online that complain about how it does (or does not do well) certain things. Haven't paid attention, but I think it has to do with how it connects to the internet. It's great for streaming movies, etc. but may not be as good for doing what I spend most of my time on -- reading forums and Googling for information online.