I didn't purchase Logos 4 JUST for the iPhone integration, but it was a part of my decision. I am very disappointed in the speed on the iPhone.
I've been using Laridian's PocketBible for iPhone since its release and, although there are certain design decisions made by Laridian that I don't like, the app itself is very fast. What I don't particularly like is that Laridian chose to ignore the iPhone UI design guide and has an app for the iPhone that behaves like no other. Unfortunately.
So, I like Logos' design better, but I find the speed intolerable.
I will use Logos on the PC and PocketBible from Laridian for the iPhone.