I am also still using an S2, My first one was ruined when I was out cycling in the rain so I got a replacement on my insurance. The battery has become increasingly poor but I recently bought a new one and if I am out all day I carry a spare, but as yet I haven't actually had to use it. I have also tried a few apps that claim to preserve battery, the latest is called Battery Saver and seems quite good.
When my contract ended earlier this year I wanted to upgrade and considered the S4 mini, I would love the S5 but it would cost me at least £30 a month, probably more, but when I realised I could get a SIM only deal that easily covered my average usage for £7.50 a month I decided to take that until my S2 dies - it is still a very good phone even if it is a bit ourdated.