Mechanical pencils are extremely useful. They don't get shorter when you use them, and they can be refilled with different kinds of pencil leads. Here are three of the most interesting ones I've used.
The Pentel Graphgear 1000 is a cheap and straightforward pencil. It has a retractable lead pipe, which means you can take it out and about without worrying about it getting bent out of shape.
The Uni Kuru Toga Roulette is good for long writing periods. This is because it includes what Uni calls the "kuru toga mechanism", which subtly rotates the pencil lead as you write. This makes sure you keep writing with a sharp tip. Some people think this removes control from the writer, but it makes it easy to write and write without having to turn the pencil in your hand. You do still have to click the pencil to advance the lead.
The Pentel Orenz Nero is made to write with extremely thin pencil leads (though there are versions that take normal leads). It has a sliding lead pipe which protects the lead from snapping. The Nero is a special version of the normal Orenz pencil, which includes the ability to auto-advance the pencil lead. You don't have to click the top of the pencil to add more lead. You do have to turn the pencil as you write it, but with a 0.2mm pencil lead you probably don't need to.
Pencils still have their place. Compared to pens: