1. I'd say that buying this is like transforming your old TV into a smart TV.
2. It's really easy to install, just plug it into an HDMI port of your TV and then connect the power plug, then once you're connect to Wi-Fi - you're ready to enjoy.
3. It automatically detects your account and it will sign you in.
4. The remote is awesome, its really compact and fits perfectly in everybody's hands. Few people on the reviews are complaining about the remote's battery consumption is very high and it lasts for only 3-7 days but I didn't find anything like that... because mine is still working for more than over 2 weeks.
5. Not just prime video but it also has other apps like Hotstar, Zee5, YouTube, etc. You can also use it to cast YouTube to TV which is the greatest part. It also has web browser but it might be a pain because of the remote but you can use the phone's remote app too which will solve the typing conflict.
So overall it was very worth buying this, got it at only 2799 during sale few days ago.
Here are few cons....
1. There is no "power off" button in this thing so you've to directly remove the adapter from the wall socket. For example you're not using adapter and you've connected the fire tv's power source directly to your TV's USB Port then turning it off will be a big problem...