Congratulations! 
ETA: Would you mind posting a list of the documents you brought with to the appointment? I have the required list, but I'd like to be prepared and would rather take too much with than be missing something once there, thanks!
Sure. Here's the approximate list (the biggies).
Me (USC applying for FLR)
*Application (make sure you put a post it with your reference number/appointment number)
*US Passport (with financee visa stamped), Expired/Previous US passport
*Few generic bank documents that showed that we share an address in the UK
Bank statements from the US (brought them along, they didn't ask for them)
*2 passport sized pictures
My civil partner (UKC/my sponsor)
*UK Passport
*Bank documents that include 6 months (you only need 3 I think) of checking account/saving account statements
*Our UK house mortgage paperwork
*My partners employment verification (payslips for 6 months)
*1 passport size picture
Important to note, I know Croydon says to bring photocopies of everything, but we just brought the originals and they happily made the copies for us. Also, we didn't do any covering letter as some people suggested. I think it makes it a lot easier if both people are there. Surprisingly they didn't ask a lot of questions. Just be sure to have everything easily marked, I think they appreciate that.
We had a boatload of documentation from our proposed fiancee visa that we applied for earlier in the year, including letters from friends family, pictures over the course of our relationship, copies of over 1 dozen airplane tickets, emails, skype receipts, etc. All of that info we brought, but none of it was requested.
Hope this is helpful! The process is pretty stressful up front but to be honest, having just gone through it, its mostly boring/waiting around, and the immigration officers were genuinely friendly and nice. The whole process from entering the building to leaving was probably around 4 hours.