The Fire Phone is an excellent device in 2015. The hate when it launched was justified since it was too expensive and exclusive to AT&T, but the device today is a much better value.
It's very easy to install Google Play on the Fire Phone and you should have access to all the apps in the store with the exception of launchers that don't work. For the $180 price tag the Fire Phone is a steal given the hardware inside of it.
Lets look at competing phones below to help you draw an opinion.
-Moto G 2nd Generation ($140). This phone features a 5" Display which is larger than the 4.7" display the Fire Phone has. The CPU is significantly weaker because it is using a low-end Snapdragon 400 and it also only has 1GB of RAM compared to the 2GB's of the Fire Phone. Additionally the camera on the Fire Phone is significantly better (13MP to 8MP) and the speakers should prove better due to the Dolby Digital Sound inside the Fire Phone. The Fire Phone has 32GB's of storage that cannot be expanded while the Moto G has 8GB's with Micro SD Support. The Fire Phone has a more premium design with a Glass/Rubber Body design although this makes the Fire Phone slightly heavier in the hand. Finally the Fire Phone should provide better battery life due to it's 400MaH larger batter and smaller display.
-iPhone 5. Short version is I can't recommend buying an iPhone 5, especially if you buy one used. The iPhone 5 is simply too expensive given competing (Non-iOS) devices that are cheaper and offer significantly better hardware inside of them. The reason I would avoid buying used is you'll never know what could be wrong with them and the iPhone 5 had a very widespread defective battery issue.
-ASUS Zenfone 2 ($199). The ZenFone 2 offers a great device at the $200 price range. It's a very comparable device to the Fire Phone hardware wise however there are some things that should be noted about it.
For starters the screen size and overall size of the device is much larger at a 5.5" display compared to the 4.7" display on the Fire Phone. The Zenfone is .50" taller, .40", wider and a thicker device with a entirely plastic build quality and is slightly heavier than the Fire Phone. The Zenfone 2 display will also be less sharp compared to the Fire Phone because while they both have 720p displays the Zenfone having a larger display with stretch the pixels out more making the pixel density lower.
They both feature 2GB's of RAM. The Fire Phone (Arguably) has the better CPU because the Snapdragon 800 is clocked higher than the Intel CPU and also because I don't think all Android Apps/Games run perfectly on Intel's CPU's due to them being 64Bit.
The Fire Phone has 32GB's of Storage while the Zenfone 2 has 16GB's of storage and a MicroSD Slot for expandable storage. I honestly can't say which device has the better camera, but if I had to guess I'd say the Fire Phone because it was built with the Camera being a large focus. The Battery Life on the Zenfone 2 should prove better being larger due to the device being an overall larger phone. The Fire Phone has a much better build quality and the speakers will also prove to be a lot better.
The Zenfone 2's biggest weaknesses are attributed to the poor build quality, poor speaker quality, odd button placement, and the crap ton of ASUS bloatware that cannot be removed.
The biggest thing you need to consider is if you can handle a phablet style device and if you'll be able to use it day to day. Personally I used to own a phablet style device and sold it because it was too big and inconvenient for what a phone was supposed to be for me. I couldn't comfortably use it in one hand and it was often tight in my Khaki/Jeans which made me worry it would bend or snap. Running/Walking was also really annoying with such a large device in my pockets.