We work it like this, two joint accounts, a 'bills' one, with all council tax, mortgage, electricity set up to come out of, and one 'current' which we use from day to day to eat, socialise etc.
My wages go straight into the 'bills' account, with my husbands going into the 'current' account (he gets paid weekly, so it's easier that way).
Any money left over at the end of the month in the 'bills' account goes straight into an ISA, give or take a wee bit so as we don't go into the overdraft.
We've worked it that way since we moved in together, well the ISA is pretty new, (got to the stage we weren't saving anything!), and it has worked very well for us. When either of us has changed jobs or such we switch who pays their wages into the right account to make sure there will be enough for everything.
But I know my sister and her beau keep their money separate, and they seem quite happy. I just personally find it odd when they talk about 'my' money and 'his', surely in a couple it's 'ours'? But then maybe that's just beacuse I've been with my hubby for so long. Each to their own.