I'm one of the few in my area who will play aliens only. Won't touch marines or ig, not even Chaos.
There was a time when I had a full army of each and every xenos race (in a few cases I had several armies worth) and I was happy with that. Once or twice I attempted to build a marine army, lost all interest halfway through (often sooner) and sold the lot.
The one advantage I believe marines have is from a modelling standpoint. They are ridiculously easy to assemble and convert, simplistic to paint and not hard to learn to game with.
I thought it was just me, but most xenos armies just don't seem quite as "user friendly."
Nonsense is our Salvation