I use iphone 5s as my secondary phone. I prefer it more than others because its size is small and easy to handle and carry. it havnt ever crashed and still have a good battery life. I have bought it 3 years before.
•who should buy it?
I think people who use thier phone for buisness purpose - to make urgent calls and to use importand social network for chatting ( some might not like it for chatting because of its small size)
•who shouldnt buy it?
I guess people who want more entertainment out of phone and want variety of apps should not go for this