Angels are beautiful fish....and I highly recommend them....but they have their drawbacks too! The first is that they need a tall tank rather than a long one. A 20g tall will be good for one or two angels. If you are thinking about having more than two then let me tell you a little story.....
When I first got my 55g it was freshwater (I didn't have a clue about saltwater back then) and I wanted to have angels in there. I already had a mated pair in my 20g tall, and I thought I would just pick up 4 more angels of roughly the same size and add them to the 55g all at once (after cycling of course). So I did just that. What a mistake!!! All out war broke out as all 6 of the fish paired up within the hour and the three pairs decided that they each wanted over half the tank!!! The fighting went on and on and I tried everything to stop it. I rearranged the decor to break up the tank into 3 kinda seperate areas, I removed the most aggressive fish, but then it's mate was left alone, removed that one too and the remaining fish decided they couldn't stand each other....it was endless and devastating....finally I couldn't stand it anymore so I took the new fish I had bought back to the LFS. With my original mated pair in there along with other fish (tetras mostly), the only time there was a problem was when they laid eggs. Then the other fish were stuck at the other end of the tank but that was about it.
Even if you buy several angels at a young age and let them grow up together, there is a very good chance that they will start fighting as they pair off. (If the tank is big enough for each pair to have roughly 2 1/2 feet of space than it should be okay!)
Other than that, they are very interesting fish and it's fascinating when they have eggs! The fry are not that hard to raise either....lots of fun!
One more thing, it isn't possible to sex angels until they are actually getting ready to breed. Their breeding tubes are how you can tell, unless of course you actually see them laying the eggs!

The breeding tube of the female is short and round while the male's is longer and more pointy.