I’ve been using the Ticonderoga Sensematic Auto-Feed Mechanical Pencil (0.7mm, #2) from our college bookstore for awhile now. The difference between this and other mechanical pencils is that the lead automatically advances as you write. No clicking or twisting needed. This is a great benefit as the lead doesn’t accidentally advance while trying to erase.
Additionally, I normally break 0.7mm leads, but not with this pencil — the lead is kept short as it automatically advances, minimizing breakage. And while I like the looks of the pencils that have been previously reviewed on this site I’m not paying $16 or $17 for one since I am notorious for losing them!
[Though it is not marketed as "refillable" many people have reported being able to refill this with standard mechanical pencil lead.--OH]