I have grand prime from past one year, as there were less budget phones in 2014 it was a good buy.it performed good for 7 months as kit kat was running very nice on it.
Lastly when it got updated to lollipop things changed , there were some tweeks which helped, but samsung added knox to that firmware which made it difficuilt for rooting the ui started lagging and memory mangement was not good.
It was a bit cramped.when I rooted it and overclocked it helped a bit, memory mangement remanes the same, ui crashed, apps were incompatible in play store.
I noticed that camera software had improved, it was fast in capturing pics in day light.the ui was not optimized as good as kit kat verson.But samsung has completely ingnored the phone and did not lower the price.we get better phone in this price.
Thats why I use this phone as backup phone in emergancy situations it works fine for me.If u already own this phone u can root it and use it as secondary phone.