Apple’s Time Machine is super.
Except when a really nasty bug is triggered whereby Time Machine silently excludes your volumes from backup. A list management software bug exists in the Time Machine volume list— when a volume is unmounted, the exclude list is corrupted, dragging some (possibly mission critical) volumes into the exclude list.
This bug is extant as of early December 2012 in OS X 10.8.2 Mountain Lion (and almost certainly on Lion as well). A bug has been filed at Apple.
Backup software necessarily must be held to a far higher quality assurance standard than other non-essential feature. Time Machine drops the ball here.
It is worth regularly verifying that TM has not auto-terminated your assumed backup strategy, and a very good reason to always have at least two external clone backups stored safely away from the computer (my favorite backup drive is the OWC Mercury Elite Pro.)
Relying on Time Machine as the only backup is an idea sure to cause painful loss of data at some point (if for no other reason than all the perils to your data). Time Machine is good for short-term stuff, but a poor solution for any professional (by itself, I do use TM for short-term protection).
Two external backups stored safely away from the computer is the absolute minimum any professional should consider acceptable.
Why do bugs like this happen? Perhaps because Apple increasingly thinks Macs are entertainment accessories with one drive (only), rather than a tool for getting work done (as prima facie evidence, Apple doesn’t even use the proper drive/disk versus volume terminology).
The wise course for backup is never assume and redundancy is mandatory.
For those with data-integrity concerns, especially over time and on backup/restore scenarios, see diglloydTools IntegrityChecker.