Note: This review addresses v3.03 of the software.
CrystalDiskInfo is a utility program which tells you just about everything about the hardware performance of your hard disk--including how likely it is to stop performing in the near future. It uses the S.M.A.R.T. (Self-Monitoring, Analysis, and Reporting Technology) protocol, which is included in most hard disks likely to be found in recent systems, to tell you what's going on with your discs. This includes such details as current drive temperature, number of failed read or write attempts, how long the drive has been running and how often it's been started and stopped, and so on. As various factors approach thresholds of danger, CrystalDiskInfo will alert you, letting you know it's time to make backups while you still can. Generally, once a drive starts to die, it will tumble downhill fast, so if you begin seeing an uptick in failure rates, it's often best to assume the worst.
CrystalDiskInfo uses a straightforward interface, showing all of the data it can retrieve from each drive on your system. (Different drives implement S.M.A.R.T. in different ways and not every drive tracks or reports every possible value.) There aren't a lot of options or settings or deep drill-downs--what you see is pretty much what you get. There also isn't any explanation of what the particular values mean--some are self-evident, such as temperature, but others, such as "Current Pending Sector Count", are not things most computer users, even fairly sophisticated ones, are likely to know offhand. CrystalDiskInfo's "documentation" on S.M.A.R.T. is a link to Wikipedia.
A free utility that can give you a heads-up when your disk begins to fail is certainly worth the price. It should be noted, however, that while S.M.A.R.T. can predict some disk failures, others can come very suddenly. There is no substitute for regular backups.