I've been keeping a diary for the last 8 years or so. The act of writing about and reflecting on each day is useful - but it's also very helpful to be able to look back on what your daily life was like in the past. It's easy to forget what you were really thinking about several years ago, and a diary is a great way to give yourself some realistic perspective.
Brief pitch: A big problem I had in getting started with diary keeping was that I'd get busy and not write anything for several days, and then I'd have trouble remembering what I did, say, last Tuesday. Sometimes, to be honest, I forget what I did this morning. I gather this is a common reason for losing the diary-keeping habit.
A few years ago I scratched my own itch on this and built "Remembary": an iPad diary app that collects photos, tweets, Facebook updates, and RSS feeds and organizes them by day, giving you some context to help you remember what you did each day. It's a bit like the iPhone app Momento, but iPad-native and focused more on writing rather than just collecting feeds.
If you're just getting into diary writing or have tried before but have had similar problems getting traction, you might want to check it out. I'm wrapping up a major version 2.0 update that should (hopefully!) be out in a few weeks. You can find out more at http://remembary.com (I'm about to revamp the site too).
It's cheaper than a Moleskine (if you already have an iPad anyhow), is easily searched, and can be backed up to DropBox or email, too.
/pitch