I was in Canada already as a visitor before getting married. The difference though, is that once we decided to get married (we hadn't yet decided when I had come to stay) we chose to do it quickly. We had planned a ceremony for about a month later (so Nov. 2010), and only invited close family. When I went to the border to get my visitor status renewed (I didn't know about extensions at the time, and we're close to the border anyway) my then-fiance (now husband) and I informed them that we were getting married 2 weeks later and then starting the PR process shortly thereafter. They actually gave us a hard time about it (lots of questions, wanted to see proof of funds, looking back they were definitely asking if I had ties to the U.S. still), and at that point only gave me a 3 month entry. I was told repeatedly that I had to bring proof with me the next time around that we'd started the application process. The next time I went was before we'd sent the application in, and I didn't know you could apparently pay fees a long time in advance, so I just brought with me copies of our marriage certificate, marriage contract (a Quebec thing), my forms completely filled and signed, copy of the paper I got from the DMP after my exam, and explained that we were just waiting on my FBI check to come back and the file would be sent in. Again, they were not super happy with this proof, but they gave me another 3 month entry. I think the next time I went it may not have been showing up in their system yet, so they only gave me another 3 months (seeing a pattern here yet? haha). When I went back after that, which was I believe September 2011, the application was finally showing in their system, and they were much friendlier (wishing me good luck, telling me they hoped my PPR came soon, etc.). They gave me a full 6 month entry at that time, which covered me up until I got PPR and landed.
I wouldn't say they were mean at all at the border (really they weren't), just appropriately skeptical of me and my status. Essentially they were doing their jobs. Haha.
My one concern for you is that you've already been in the country a significant period of time and you're asking for an extension to stay another 6 mths so you can plan your wedding and get married. I know when I saw them just two weeks before my wedding they were skeptical, and I'd only been in the country 6 mths at that point. I'm just concerned that they will deny you an extension, since you're essentially asking to be allowed to live in Canada for close to a full year before getting married and making your PR application. This was why I counseled you on your other thread to consider getting married sooner, especially if you aren't planning a big wedding. For the record we had about 10 people at our first wedding and it was no less special than the 100 person wedding we had 8 months later.
Of course, all of that said, my experience is just one out of thousands. I cannot claim to know what CIC will say when you send in your extension, and I think for $75 it is definitely worth doing. I just would make sure to have a plan B in place in case they inform you that your extension has not been granted.
