The narratives (sponsor and applicant) are important because they are the only opportunity you normally get to tell your "story" to CIC. The evidence you present is corroborating that story, but the story is really key here - why should a suspicious, jaded, "seen it all before" visa officer believe your relationship is genuine?
Having a story from just one party isn't as convincing as having a story from both sides. I prepared pretty much everything in our application, but I had my spouse write a narrative. I asked a friend (who is a professional editor) to provide light editorial guidance on it as I didn't want it to sound the same as mine. I wanted the visa officer to read our two narrative and get a sense of who we were - and to know that those narratives were written by two different people with strong feelings for one another.
As for the supporting evidence, our application followed the standard advice here: put it in a separate section and reference to it. We had lots of different types of evidence - old chat logs, e-mail exchanges, photographs, evidence of trips we'd taken together, documents that showed we had intertwined our lives - a single lease with both our names, a joint bank account, our jointly owned car. Some people put hundreds of pictures in the application - I think we did 20 or so. But when I was done looking at the narratives and the evidence I found it was compelling - someone looking at the application would get a strong sense of a genuine application.
For my application it was also imperative - my first application (as a skilled worker) was denied. That is a HUGE red flag. The saving grace is that I met my spouse just before that first application was submitted, let alone denied. The first officer who looked at our file in Ottawa was happy with the file except that one issue - "Based on information provided, further review required due to previous in admissibility". But that officer also said: "Eligibility: Photos of relationship provided, copies of e-mails also provided as evidence of communication. Relationship appears to be genuine." That told me the application achieved it's purpose.
It actually worked better than I'd expected: I'd been prepared for an interview (rare for US applicants, but it does happen) and a drawn-out process. Instead I was the first US applicant in my month (March 2012) to receive PPR.
So, my advice: write your narrative. Explain to the officer why your relationship is a genuine one. It does make a difference.
I cry when I read the letter my spouse wrote, even now, a year later. I know that helped make a difference.
