I personally haven't read any, but my understanding is that they're hugely popular in urban fantasy right now. To the point that some agents are saying "oh dear god, please no more angels." I think the standard plotline is something along the lines of "angel is stuck on earth for some reason and ends up having a relationship with a human while trying to save the world from demons."
That said, I don't think there's ever any reason to avoid any topic as long as you can put a fresh spin on things. It probably is in your best interests to read widely, though, so hopefully someone with more knowledge in this particular area can giv eyou more book recommendations.
I came up with, what I thought would be, a great story line about the war between angels and demons but after doing some research I found lots of stuff out there :-(
However, I do believe that you should write about what interests you regardless of what's out there otherwise you run the risk of getting bored or losing heart with your own project!
Go for it and see where it takes you but look into what else is out there so you can see if you are putting your own spin on it.
thanks every one for your feedback. I have been looking up stuff just about all night and I think the spin I put on my plot will be different. there are going to be elves, dwarves ,dragons & dragon kin , humans and a few others. plus my series will be geared more for adults(sex, drugs and lots of war and gore)