The battery may only be 1430mAh, but with components that require less power that battery goes a long way. You can get up to 10 hours of talk time on 3G, or up to 51 hours of music playback.
Being a Windows Phone, the Lukia 530 doesn't have access to the Android or Apple app stores. The Windows store doesn't have nearly as many titles as other operating systems, and usually updates roll out slowly (if at all).
For basic social media and music playback, the phone performs good enough. There's some touch input latency when interacting with the display as well, but for $20 I don't think you'll do much better.
As you'd expect with a $20 phone, you get what you pay for. The 3.4" screen is a plastic 320x480 display, there's only 2GB of ROM and 512MB of RAM. The 2MP camera is barely worth using.
This is actually a $100 phone which is subsidized by ads on the lock screen. That means that you're getting better hardware at a cheaper price, and all you have to do is put up with ads on your lockscreen. By default there is also an ad widget on the homescren, however this is easily removed.
All ads are provided by Amazon themselves, and not Blu. Each time you press the power button, you'll get a new ad so you aren't even seeing the same ads over and over.
Currently, ads can be removed if you install an alternative lock screen (downloading one from the Google Play Store works). This may be removed in a future update, but for now this 'hack' is working.
This device can take 2 SIM cards at the same time. This is important for some users (especially in Asia and Europe) who require 1 SIM for work and 1 for home, or perhaps 1 for data and 1 for texts/calls.
Blu is a budget phone manufacturer, and they are known to update their phones very slow, and cut support early. This phone likely won't be getting many future updates.