Honestly, $50 smartphones tend to suck a lot. Even the highest-end Firefox OS phone for a while had major keyboard lag. Chances are trying to use things like Skype/VOIP over them would be painful....
I think if you're looking to invest in one useful thing, a good phone would be a great purchase. Maybe the Moto G at $200?
Amortized over a year, you're talking about a lot of bang for the buck.
I still use Samsung Galaxy S3 with Cyanogenmod I bought for 60€ that still meets my needs. If you are on a budget, buying a new, shiny smartphone is a waste of money imho as you can get a cheap, used but fully functional one for much lower price.
So do i on Cyanogenmod 10 (Android 4.4.4) and it handles all web/music/video needs. I guess the latest games that you wouldn't really want to play on a mobile anyway might have issues.
The S3 generation of hardware has stood up very well.
My only reason to upgrade would be for some good security hardware that can support full disk encryption with no slowdown.
