Very nice. I just finished my first AR recently, I used the same DPMS upper as you have there. I like it alot, had a 4-5" group using open/flip up sights at 100 yards standing, which is really good for me. No malfunctions/problems yet. I don't have an optic yet, I opted to get back up sights first. I have a set of Yankee Hill Machine flip up sights on mine, they make a front sight that will align correctly to go on that gas block rail. Mine work well. There are cheaper ones than the YHM though, but make sure not to put a polymer sight on the gas block(or at least that's what I have read, didn't try it). You probably already know that though.
Anyways nice build, Thanks for sharing!