The $50 version of this phone is an Amazon Prime exclusive, and it comes with ads on the lock screen. A similar model is available with no ads for $100.
There is also a higher end version available for $60 with ads, or $110 without ads.
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Pro Ads are removable
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.
Pro Dual SIM
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.
Pro Decent hardware subsidized by ads
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.
Con Very little storage
With just 8GB of storage and 1GB of RAM, you will likely require a SD card slot and clear out recently used apps for a decent experience.
Con Poor update history
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.