Mine for the 1D X. Reminds me that I need to set this up on my EOS M.
Format card: I swap out the card in slot 1 after almost every shoot, so I prefer easy access to re-format.
Battery info: Good to know charge remaining in more detail than the battery icon.
External Speedlite control: A bit easier than changing settings on the ST-E3-RT; with three 600EX-RT flashes, the three groups are sufficient for my needs most of the time.
Selectable AF point: I usually have this set this to only cross-type points for selection speed and focus accuracy, but with an f/5.6 lens (70-300L, 100-400L, or 600II + 1.4xIII), I often want to be able to select points away from the center columns.
Save/load cam settings on card: I have different groups of C# settings, e.g. people, sports, birds, etc. They're saved to each of several CF cards that I keep in various camera bags, and I load the set that's needed for my planned shoot.
Restrict shooting modes: In some of my settings groups, C1 is for tripod use (MLU, timer, etc.), while C2 and C3 are for non-tripod use. I disable C1 when not needed, so the M.Fn button can more quickly cycle through the settings (it goes from C2 to C3 to which ever of Av/Tv/M was last used, so it's one button press from C2 to C3 and two button presses back to C2).
I do have the Start from My Menu option enabled.