"...part or all of their backup solution."
USB HDDs for part of the solution is okay. For all of the solution, it's not a good idea.
When you plug in your USB HDD, your entire backup medium is now online and writable, on the same machine which has your primary data.
In the event of:
- a glitch in your backup software
- an electrical failure
- a virus
- human error
- fire or flood
You could lose all your data, even your backups.
One poor-man's solution would be to use two HDDs, keeping one off-site and off line (e.g., at a friend's house).
The disks are reliable. Some USB electronics are flaky, so test your backups.
