That is a very good question and since this is my first SS bike, but I have done a lot of e-research from this forum and roadbikereview, as well as other bicycling reviews.
I was looking at the Capo, Langster, The Bikes Direct lot, Bianchi (not Pista but Roger) and it came down to a couple of things.
Price, while it is not the cheapest when comparing to the bike direct offerings, in terms of the Capo, Fuji and other 'name' brand bikes; it is on par with the others. If not a bit less expensive.
Weight, I noticed that when comparing the SS bikes, a lot of them weighed over 20lbs and I wanted something a bit less. While I don't know the Mark V's weight, I've read that the Jamie Roy (alum.) is around 18lbs stock.
Another reason I went with IRO is that it's a small business. I enjoy the fact that it is a small business. I for one, ran a small BMW business and enjoyed many facets of it. It is not a corporation, it is not a branding issue, its what I tend to believe heart and soul.
That is why I went with IRO and I can only speak for my self.