I has this phone in 2007, and I must admit, it was a solid, well performing mobile.
The only thing that put me off it, was that this was the 2nd Sony Erricson in a row (previous K750i) that the dedicated side camera button failed to function. It wasn't a big deal as you could always take piccys using the central "joystick" type control, but it did put me off these phones as it happened twice on two separate handsets.
Yes, I would maybe try another battery - might be your best bet. As someone else points out, if you've been taking piccys on the memory card it's worth backing this up on a pc (I always did this)