I like both The Decemberists and Neutral Milk Hotel.
I say, the more the merrier. and they do have a unique sound,
it is just in the same genre.

Yeah...I sensed the same thing when I first heard them. I felt that sense of being torn between admiration and manipulation. However, I think the Decemberist have their own tone and aesthetic going; a sort of sea-ferring, Flannery O'Conner meets Bright Eyes in the lime light kind of thing. I find it kind of precious, but convincingly addictive.
They pass the test in my book.

True, I was always told "If you love Neutral Milk, you'll like the Decemberists." I picked some up and really all it made me feel was that same sad longing for more Neutral Milk Hotel. I generally ended up popping out the Decemberists and popping in Aeroplane Over The Sea. I still listen to the Decemberists now and then and try to give em a chance, but for me they don't do much. There's a vocal similarity, but the lyrics bore me, the music is alright, but not as dynamic as NMH. Even without comparing them to NMH, they just fall short for me, but I can see where they fit into their own little niche and appeal to a lot of people.