Myself and a friend both purchased Galaxy S3's about the time they were released in the UK. So now they are around 2 years old.
Over the past 3-4 months, I've noticed my phone has been getting much slower. The most annoying thing is that when a call comes in, I would swipe the "answer" button, but it would freeze and I would end up missing the call.
My friend also described having the exact same problem. My girlfriend starting reporting that my calls were very "fuzzy", and she couldn't hear me clearly. I also noticed on a couple of calls, that the caller was struggling to understand what I was saying.
I did a factory reset and honestly, I don't see much of an improvement. It's quite annoying as 2 years isn't really a very long time. I know people upgrade their phones regularly, but I think they should last at least 5 years or more.
Anyway, I'm thinking of upgrading to an HTC One which I've seen has good reviews.