I actually started on Y:The Last Man when it first came out because Vaughan pitched it on the monthly What's Up at Vertigo Page (the one that appears in every single Vertigo comic) when I was reading Vertigo Pop! London at the time as a "feminist" work even if it isn't obviously feminist.
I had to drop it though, because my comic budget was already begging on its knees. The first arc kicks off very excitingly, with a lot of intrigue, and the character moments and dialogue are great. The thing is, for me, little is resolved by the end of the first arc that it made me disappointed even though the experience was enjoyable.
But the way it really addresses a lot of points and issues within a radically altered socio-political landscape is really good, and I find that this is Vaughan's specialty, even if you're reading something as "political-lite" as Runaways instead of Ex Machina.