I shot my sisters wedding as the only 'official' photographer a few months ago, I thought long and hard about it (never shot people, let alone a wedding before) but as she had no plans to hire a pro I decided I may as well, nothing to lose and everything to gain.
My best advice would be to shoot what you're comfortable with. Everyone said borrow, beg or steal a 24-70 2.8 / 70-200 2.8 combo to make sure everything was covered, but I love my 35mm prime and I'm so, so pleased I stuck with just that for the vast majority of shots.
In terms of missing out on the day, if anything I thought the opposite, I was there for everything from start to finish, my own wedding aside I've never felt so part of or enjoyed a wedding so much. I'm pretty shy and don't drink, so being forced to be heavily involved actually helped me come out of my shell. On a practical level it was very easy to arrange me being in photos, normally one of the best men would take the shots I needed to be in once I had all the settings correct.
I did everything for free including the prints, CDs, slideshow etc (as well as invites, place names, table plan, basically all the stationery), loved every second of it. I LOVE taking photos, and I love designing stuff, so any excuse to do either of those I would normally jump at. I've read so much about the whole 'taking work from pros' argument, but as I see it I'm getting to do something I am unbelievably passionate about for somebody I love. I would do it again in a heartbeat for a close friend or another member of my family.
For the record my sister and both sides of the family were unbelievably grateful and impressed with the photos and my attitude/method during the day, so it was a win-win in the end. The only downside was my wife thought the shots were better than the £1k pro we hired for our own wedding.
My favourite from the day...

First Wedding by
Harry_S, on Flickr