Well, I got my Jawbone headset last night. And after using it for all of 10 minutes, it's going back to Apple.
Firstly, as others have said, I couldn't get it to fit my ear correctly. Yes, it is big...bigger than other BT headsets. The earloop just - any of them, lef tor right - just doesn't fit my ear correctly. It falls off WAY too easily, feels like it's sticking out way too much, and all in all, just doesn't "feel" right.
But the deciding factor? The sound is TINNY! (That's not "tinny" as in small, it's "
tinny" as in metalic-sounding.) Where's the "crystal clear" sound the Jawbone is supposed to have? I placed a few calls, and recorded my voice using it (with both the noise-cancelling feature on and off; that makes a difference, but either way, it's still VERY tinny), then listened to myself. Not impressed at all.

My Motorola RAZR H3 Bluetooth headset sounds clearer, and it's a year old! Plus it fits much better.
I've got to call Apple today to exchange it, and I'm going to go with the
Apple iPhone Bluetooth headset. It's $10 more, and hopefully they still allow me to (re)use my $100 store credit, which means it'll cost me $32.09 for it.

Jawbone.