I have an FC12, and kinda wish I had an 18
12 switches gets you 8 scenes, next and previous presets, tuner and tap tempo, very cool. However, I also want to control chorus, one or [EDIT]
two more drives, delay, rotary (fast/slow as well as on/off), phaser, flanger etc. I've spent a lot of time thinking about it, not super happy with anything I've come up with.
I'm using hold actions to switch layouts, but that's pretty awkward to do between the verse and the chorus, and then you have also actually turn the block you want on, and undo the whole thing to get back "home".
[EDIT] I'm also using hold actions on most effects page switches to increment that effect's channel. But again that's awkward, especially since you can't see which model each channel uses. (Pretty sure there's already a wish to improve that.)
I know these are 100% first world problems, but at the very least, I'm not considering an FC6, unless it's in addition to a 12. Consider yourself lucky if you're happy with a 6