I like to keep my descriptions pretty simple. (salty, sweet, leather,cocoa, etc) but I have to admit, I've tasted and used the descriptive 'pencil lead' for cigars many times.
As others have mentioned, its sort of like the smell right after you sharpen a pencil, not necessarily the lead itself. It's not an unpleasant taste either, unless its really overpowering. I think the only cigar that I remember having a predominantly 'pencil lead' flavor was an RP edge from like 3 years ago.
yucky.