I've used a mechanical pencil. There's not a lot of pressure on the pencil, so you have to use a lead that's soft, but not so soft that you have to constantly pause the drawing to advance the lead. Alternately, you could use an abrasive drawing surface, like gessoed paper, but then again you'll need to pause often.

I never got a pencil drawing I was particularly happy with. I never tried a pencil with "scribble" mode, though, which might be good.

I've also been unwilling to sit for hours, slave to the pencil, which is what it would take to do a large drawing.

Presumably a soft pencil would blunt slightly over the course over a dense picture?

I did a few tests with pencils when I first got started. Mechanical pencils certainly gave the cleanest and most defined line. Thought I would have to be clicking the lead every few minutes but was pleasantly surprised how long they lasted. True I was just testing things small scale so for large work this might become more of an issue. I think the softest lead I found was 4B for a mechanical pencil and using this with a little more of a tilt to the board gave a nice clear mark.

Going the route of your traditional sharpenable pencil gives more options for softer leads still and charcoal/pastel pencils too of course. The line with these was less defined and varied a lot more as the lead wore down but I quite liked that myself.