The iPhone in particular is very well-thought out. For example, they aren't including support for 4G LTE until they invent a new battery that will still last all day while connecting to 4G LTE. My friend just got a leading Android phone that has 4G LTE, but it runs out of battery after 6 hours.
I would recommend checking whether it is well-thought in the area you do care about.
I was surprised to find out that you cannot easily save PDF from a webpage to reliably keep it on the phone forever; it will be subject to cache retention policy. There are more obvious limitations, of course. Nobody is free of mistakes, so check what is obviously important for you in the specific device you are going to use.
I have often benefited from recommendations for Things I Didn't Know I Wanted.
Given that Less Wrong is a community of unusually intelligent, critical, and self-improvement-focused people, I suspect we can generate a pretty helpful thread of product recommendations — perhaps even a monthly thread of product recommendations.
Rules: