I'll also add that it while it always keeps your most recent backup, it also keeps "older" backups (up to a certain amount of time), so if you did something stupid - say selected all the text in a document, hit delete and saved it by accident and let it sit for a few days - you can get more than likely get an older revision of the document.
The client software installs a explorer extension that lets you right click and restore files VERY easily, and you can also copy them off a "virtual" mozy drive.
All in all, it is a really awesome service. I have 3 computers backed up, with more than 90GB of files, and I think it's TOTALLY worth the 15 bucks a month (5 per computer). With digital photography these days, having a solid backup is very important if you want to keep your photos safe.