Lizzyday you said "God as a concept isn't something that any two Christians (or people of any other faith probably) would experience or understand to be the same thing, if they even claimed to have any proper understanding of what 'god' was at all."
To anwer that, if I may, from a Christian perspective, I would say that certainly I do have an understanding of God and I am seeking to increase my understanding of God.
Even thinking about life from the perspective of athiests is helping me to refine my understanding of God, so thank you for that.
I would say that no two people are the same, we are all unique, and we all experince life uniquely to us. Yet in spite of that uniqueness we learn to love each other, our families, our spouse/partner etc, to care for people quite far away who we may never even meet, and so to co-exist.
I often wonder if people see the same blue, that I see when I look at (for example) the mumsnet banner, or the same red of the flag of Japan etc. When we listen to music do we hear it the same as others, we will never know. Yet can we together enjoy music, enjoy colours etc, I would say yes. Amd I think that although there are huge differences in the way that Christians perceive God for many of us there is something that links us together.
I know for me that the idea of 'by your fruits you shall know them' sorry that is a Bible quote but I can't think what the right way to say it in an idiom would be! But basically, when I meet loving kind people who know Jesus I feel a connection. If I met someone filled with hate and anger who claimed to know Jesus I would not feel that connection. And I know, because I have been around someone like that, and I felt it was very sad indeed. I am not sure if I am making sense, it's late! 
But I wanted to reply, because as a Christian I spend a lot of time connecting with God and I connect with other Christians even though we do not agree on every single point of belief.