It's hard to discuss this topic subjectively because there are so many views. Christians believe in God, devil, angels and demons but if you mention spirits they freak out, even though they refer to the holy spirit all the time. There are many other religions that believe in many other idealogies, but I'm not familiar with them.
What I can talk about though is what I believe in and also what I see most spiritual but not religious people believe in.
"God" or "Source" as it is called I see as the canvas of the universe, it's the very fabric that allows everything to exist and is also what connects everything and determines the rules and laws of the universe. It's purely what allows reality itself to exist. I believe it's purpose above all is experiencing. I asked the question "Why existance?" and the only definite answer I could come up with is "To experience!" as it's all that makes sense. If this is true, then when we die we return to source, and we are not lost to the universe as the athiests believe.
As for entities, Erin has mentioned that Angels and Devils are just personifications of astral entities. Just like how people can be good or bad, both a part of experience, there can be positive and negative entities. You couldn't properly experience without an abundance of both, as too many of one or the other would be detrimental to experience itself. The labels of Angel or Devil just allows us to better handle these entities.
As for the "fallen" idea of angels, the world has changed and evolved, and with it a greater polarisation. Many entities that were positive might have been weakened and turned negative. Negative entities also could have been redeemed by others, or just evolved themselves and risen to become positive entities. Whether they all started off as positive, negative or neutral is anyone's guess. Perhaps that's a good question to ask one if you ever meet them.
In the end I think the three main graces of God are Existance, Experience and Free Will. This gives us the chance to be good, the chance to redeem ourselves in the pattern of the universe, live a better life and to live in harmony with what is, while at the same time punishing us for our transgressions and making it a challenge the whole way. Life isn't easy or fair, but in the end I think everyone gets what they deserve.