For me there are both positives and negatives. One big negative to not being a Christian is the whole afterlife thing. At the moment I just see death as an end of consciousness. We're alive and then--blink--it all goes black. Forever. I sometimes wish that I believed in heaven, so that I knew I would have a chance to see family and friends again, and so that I could live on somehow for that matter. I also sometimes miss the assurance that I had as a Christian. Whatever might have come, whatever might have happened, I could 1) assure myself that God was in control, and 2) believe that my prayers had some impact, and thus I always had some degree of control (do points 1 and 2 contradict each other? that's a discussion for another thread, I think).
On the positive side (of no longer being a Christian), I feel much less guilt about things in my life, which has increased my happiness significantly. I no longer view myself as a sinner, as most people would use that term. I have flaws, I make mistakes, but I only "sin" in the sense that sinning is "missing the mark". I do not think that I sin in the sense that there are spiritual consequences attached to them. I don't believe in a soul, so how could I believe that I have tarnished my soul? I suppose I am also happier in that I believe I am still striving to find the truth. I would be miserable as a Christian, because I do not believe what Christians believe. I would feel like a fake. I would feel guilty, like I was just playing church, when things like our world view and how we conduct ourselves deserves more respect than that.
I now forge my own path, I am not following the "royal road". That has both positives and negatives. It was nice to know, as a Christian, that I had guidance in the form of saints and holy books and Church Fathers and so forth. Now I'm on my own. I wouldn't say that I'm free to "make it up as I want," though, because I'm still hampered by a nagging want to do what I think is right and believe what I think is true. Schopenhauer said, truthfully I believe, that "a man can do what he wants, but he cannot want what he wants". I didn't leave Christianity because I got mad at God, or wanted to lead an immoral lifestyle, or something like that, I left because I felt that I had to. I felt that if I was honest with myself, and if I was honest with others, I had to admit that I just didn't believe. Maybe some day I will believe again. And I will return in that case. I will bounce like a tennis ball for the rest of my life, for all I know. And I don't really care, either. I would rather bounce back and forth and know that I am being sincere, than stick to one path and know that I was living a lie, not to mention feeling miserable.
There is also a certain exhilarating feeling of freedom when you are on the outside looking in on Christianity. So, overall, I am happier now, though that is mostly because I am being honest with myself. I could become a Christian again 6 months from now and be happier then than I am today. I doubt that'll happen, but I don't deny that it's a possibility. There have certainly been times in the past when I was a very happy--if struggling--Christian.