The key feature of Time Machine is hard links to directories -- which is only possible on modern HFS+ (and rsync doesn't even try). Some people like the UI too, of course.

Without hard linked directories, a full --link-dest backup of a decent sized disk, with zero file changes from the previous pass, can easily consume 100MB (and take 45 minutes to perform).

This disk consumption might seem insignificant, today, but that's 2.4GB per day if you run a standard Time Machine equivalent backup schedule. Of course you might not choose to do that, because the previous hour's backup would only finish 15 mins before the next one started, which is insane.

These numbers are from direct experience on a 2TB, approximately 60% utilized source drive.

That said, I use rsync, not Time Machine, for my OSX backups. You'll want a few additional switches for HFS+, and if your target drive is HFS+ also, make sure you turn OFF "ignore ownership on this volume" in the Finder... but the script posted here has the right general idea. Somewhere on my project list is adding directory hardlinking to rsync.