Dunno, I’ve been using Firefox since before it was Firefox as my primary browser and I’ve never used sync. I don’t think that’s the most compelling feature.
It's not the most compelling feature for me, but I spend 2 hours on the bus every day and love being able to send a bunch of tabs that pop up on my desktop when I get home.
It can be neat, though my problem is I've several hundred tabs "archived" (discarded/hibernated/whatever) on the phone and and I want to open them all on the desktop at the same time to go through (I'd be very happy for the 10 minute wait, and/or if they were all discarded/hibernated, as doing them one by one would be an absolute chore), but trying to kills the desktop FF by making it hang then fall over!
Sync is not a feature useful to me, either (along with most of the other newer features). Lots of people do find it very useful, though, and despite it not being for me, it sounds plausible that it could be the most compelling feature for the majority.