Absolutely worth buying!
I have had Hard Disk Sentinel for years. The report window appears every time that I restart but everything is always good, so I almost ignore it. Until I was consolidating my old backup USB drives and one 2TB USB HDD reported 88% health and I "panicked." Backed up everything without any issues and then restored the drive using HD Sentinel.
Then I plugged 4TB USB HDD drive and after using a short while it reported 9% health, 14 days of life remaining!! UGH! Ok, so I didn't need to panic at 88%
Even at 9% health, I was able to verify nearly all of the files matched my other drive (I try to have 2 copies of all files) and copy the few unique ones. The health dropped to 7% and 8 days life remaining. I am now partitioning the drive to avoid the bad sectors and then will keep using it as a secondary backup.
If I didn't have HD Sentinel, then I would have soon added new data to this drive and maybe lost it.
I think that the unregistered version will keep monitoring but you can't repair. Hard drives fail, so sooner or later you'll need to repair something. So buy it when on sale.