I've been a fan of mechanical pencils for a surprising number of years now, I started using them at school when I was about 9, and have carried one since.
If my memory is correct, I started with a green/black Staedtler graphite 777, but lost it. I then got a Pilot Shaker (H-245), which served me faithfully for a couple of years, until it was finally broken beyond repair. After that (I think I was 11, maybe 12) I got a Staedtler exactly like the first one, and that lasted 8-9 years. I decided to replace it last week. Not because it's broken, it still works perfectly, but because it looks like it's seen enough of war.
Now I have a Penol Double-Click/Knock and I'm enjoying it very much.
I've never really used anything other than 0.5 HB lead. I'll take a fine line rather than a broad anyday, and after four years of studying design, craft and goldsmithing, I've found that soft lead will probably be the one thing that pushes me over the edge. While I use softer regular pencils for sketching, the final draft and details had to be done with a 2H pencil or HB mp, due to smearing. My teacher once reduced my overall grade on a project by a whole point (from 5 to 4 out of 6) because some of the writing had been smeared in a corner. (A mistake due to haste, but clearly unforgivable.)
I no longer carry regular pencils aroud, though. My daily stash has been reduced to a fountain pen, a mechanical pencil and a few pigment liners.