The multi-material upgrade is a lot of fun. I had a minor issue at first, where the filament would repeatedly get stuck at the end of the cooling tubes. I decided to ream the filament path(s) below the cooling tubes with a 1.85mm drill, and haven't had an issue since (knock on wood). I love the simplicity of the whole thing, and the ease with which the multiplexer opens up to debug any issues.
For anyone interested in getting a multimaterial upgrade, it's probably good to know that the normal wipe tower settings don't work very well for especially black filament. In order to avoid filaments 'colour bleeding' into each other, I usually had to double the wipe tower size. In combination with constant filament switching, this means you should be prepared for very, very long prints, across multiple days for moderately large objects (with many colors). The large kirby above (about the size of a tennis ball) took 11.5 hours to print. Another thing to consider is that print quality seems to suffer sometimes, I guess because of the bowden setup, especially if a color is only used for a small detail (see for example the eyes of the small omnom above.) Interestingly, it seems that this depends on filament (color).
I will play more with multicolor prints in the future (when hopefully there will be filament available with less bleeding, as well as per-filament settings for the wipe tower!), but for now I will order some support material and have a go at printing my SNES controller with it.