3-4cm is still pretty long imo, I trim mine down to about a blade 2 or 3, think that's about 1cm.
Not yet, touch wood. ;-) It's just a little awkward, usually sit in the shower to do it and use both hands. Gotta pull the skin really tight for every stroke of the blade, then it's just like shaving your face, but I'd hate to think what could happen if just haphazardly throwing a razor around some of the folds in the scrotum ..... ouch!!