High quality piece of hardware, ruined by Amazon's software.
The phone is solidly built (and a little heavy) and that was the first thing I noticed about the phone. Then it really started going downhill. At first I liked Fire OS, but then I began to realise how limited it was. The selection of apps was very poor, so I ended up only downloading ES file explorer in order to side load Google apps. Then after a few months I had had enough. I flashed CyanogenMod 11 onto the phone and I liked that much more. Still I was disappointed with the fact that there is still no Lollipop update, even now in 2017. Amazon ditched support for it as soon as they realised this was a train wreck from start to finish.
Performance. Back in 2014 with a Snapdragon 800 it was decent for a phone but due to the clunky OS it really wasn't amazing. Phones such as the LG G3 released moments later had the 801 and they performed much better. In 2017 the Fire Phone scores 45-50,000 points on Antutu, while on the Geekbench side of things it gets around 800 on single core and a little over 2000 on multi core. Basically, if you're looking for performance this phone isn't great, but it'll still beat many phones costing £200 or more for about £70, refurbished.
The design. It looks a lot better in person than it does in pictures, and I've actually come to respect it as quite a pretty little phone. The cameras on the front give it a kind of character that no other phone gives you. Apart from those big bezels I think it looks just as good as the iPhone 7 (please don't start a riot). Build quality is actually up there with the best, even today. It's quite a hefty phone when you look at mass to screen size ratio (150g on a 4.7 inch chassis), but that adds to the great feel of the phone with its glass back and soft rubbery plastic chamfered edges. It's quite an angular phone in the hand, but not uncomfortable to hold. Buttons are nice and clicky, though I do think the home button could be a little bigger, and I would have preferred the power button on the side rather than the top, since you have to scooch your hand up the phone unless you can use the other hand, which on a glass phone could lead to disaster. Thankfully it has gorilla glass 3 on the front and back to protect it. There is also no camera hump. Yay.
To conclude, you should get this phone if you... no, just don't. Get an LG G3 Cat.6 if you only have £100 to buy a phone with, or if you want a more compact phone, HTC One M8 will probably suit you better. These phones come with a much more refined and updated OS with no gimmicks. I mean, seriously, the phone takes up to 5 seconds to load up a funky 3d lockscreen that you're just going to immediately swipe away. That's a lot of wasted time staring at a blank screen.