This makes it much easier to type commands, since you don't need to use a keyboard chord.
After using this for a year or so, I found out that the semicolon actually has its own function--it can refer to the current insertion point when writing ranges. But I've never used that; one could remap : to ; if desired.
In normal mode, after using f or F to jump to a character, ; repeats the motion and jumps to the next match. I find myself using this often enough that if I swapped mappings for : and ; I would be in a world of confusion.
I map <space> to : and it's been great. <space> is normally just a synonym for l, so I never used it; my only muscle memory issue is just an occasional : to enter command mode. Perhaps I should unmap :.
After using this for a year or so, I found out that the semicolon actually has its own function--it can refer to the current insertion point when writing ranges. But I've never used that; one could remap : to ; if desired.