I recently ran a project to convert my three main websites from single WordPress installs, into one multisite. There were many reasons to do this, it is much easier to maintain, with only one WordPress install, it means that all the sites have share plugins and settings, it means that I need not worry about old versions of plugins and different plugins on different sites. I had 3 different plugins on each site doing SEO and sitemaps for example.
The process was a pretty simple one. Create a wordpress sites, enable multisite and create the blog. From there I used the wordpress exporter / importer. I wanted a fresh install of WordPress, so the xml exporter seemed like a good fit. However, the WordPress importer really doesn’t work very well, as it failed a number of times and failed to import a number of the images. The wordpress importer is broken and there needs to be work done on this plugin if it is be used as a mass market product. If I were to do the import again, I would use the WP CLI importer, as it doesn’t use as much memory. The wordpress domain mapper, also had a lot of bugs in it, it really isn’t clear how to use it. There only good thing about multisite is I have now enabled caching through the network using the W3C total cache plugin.
All in all, I think it was worth my time to do, as in the long run it means that I can keep all of my sites up to date a lot easier, but it isn’t for the faint of heart.