The unfortunate thing is that Tolkien created a monotheistic world with a god similar to the Christian concept of God, but it aint that simple a comparison if you look at it closely.
First sticking point of course is that Tolkien was not writing allegory. His creation is a literary creation, it is an independent world, not
our world. You are free to get some applicability out of it if you find delightful or not so nice things which mirror your own experience or understanding - but Tolkien was very wise when he said that you can get no more than applicability as he immediately stepped away from the whole rigid "This equals this" type of thinking. He knew that we as readers can never truly know how his own understanding of God worked and so we can never know if this really is shown in Eru.
The other huge sticking point is that everyone has different ideas of God. For some, Eru might be exactly like their understanding, for others, Eru might look nothing like their understanding of God. And it certainly doesn't apply across the board either way to all Christians! For me, who went to church every week until I was about 13/14, this Eru is
nothing at all like the God I learned about. He bears some similarity to the Catholic version of God who my Grandmother would tell me about (I was firmly C of E), but even then, he is still fundamentally different. Eru creates Melkor who creates evil and allows it to happen, whereas the Catholic God created people and they then undergo The Fall - there is no 'Fall' in Tolkien's creation, not unless you really force some analogies.
I think the most interesting and likely comparison to Eru can be found in the Book of Job. Here, very much, God = Eru. In many ways I think Tolkien was exploring with Eru his own understanding of a God who can allow millions of people to die in the most awful ways imaginable and yet be essentially 'good'. Eru is unpleasant and certainly exercises his 'omnipotence' - he's an interesting expression of a man who had to balance a faith which he felt emotionally attached to because of his lost mother with the vivid and visceral memory of men being slaughtered in front of him in the trenches. After WWI, the church began to lose its hold on people, as they simply could not accept that God would allow this to happen; Tolkien on the other hand has this attachment to his faith through his lost mother, and seems to have gone back to an almost Anglo-Saxon version of faith where God and Wyrd are mixed up in one bloody, not-very-humane whole.
So in a nutshell, no, Eru is not
the God, but he is
aGod, maybe even
your God. If he's yours, then fine (though I'm intrigued as to how you have acquired such a Brimstone version of God in the post-Reformation era if you're not Catholic!), but he bears no comparison to other people's God.
As to the Valar - they're
way better than angels being that they derived from Tolkien's readings of Norse myth and the Gods. Much more exciting to me