We migrated four standalone WordPress sites to a new host using WordPress Multisite, one shared theme, and added in multiple custom options.
Front End Description
We worked with the client to standardize one theme across all sites by using custom admin panels, custom headers, and custom menus across the sites. We also worked with the client to add a slideshow and the road trips options on the home page.
Back End Description
We put in place a custom admin panel for the main site which controls things like the slideshow, category listing, post sorting, category specific ads, etc.
Lessons Learned/Random Thoughts
Migrating multiple sites into one created a few issues, but at the end of the day when you have one install, one theme, and lots of customization options you can generally focus more on what really matters - the content.