Here's a shot of the Plaza taken from the same spot, but with the main iPhone lens. There's no vignetting, and when you look at it compared to the previous shot, it looks like it wraps around a lot less.
This is something that happened a lot with the Revolver lens — it adds a slight circular distortion to the shots. Sometimes this makes for a really cool effect, but other times it can be pretty distracting. I like it for this, especially since the Plaza is way too big to fit in a shot taken from the regular iPhone lens.