My SE is the 64GB, it replaces a 16GB "5".
I wanted an iPhone update, but didn’t want the larger and more expensive 6 series. My SE came from an Apple Store as I traded in my old iPhone 5, which had a rather poor battery.
The SE has much better battery life than my old 5 had, even when the 5 was new. It is also much faster and more responsive. I love the fingerprint reader and Apple Pay.
Having more LTE(4G) bands than my earlier 5, in some places I get 4G coverage where I only had 3G on the 5.
I’ve updated the firmware to iOS 9.3.2 without any problem.
I have found no down sides with the SE at all.