My wife was kind enough to assist me in my quest for the perfect Bluetooth headset by giving me an Aliph Jawbone for Father's Day. I spent some time wearing it and came to the conclusion that the ear fit wasn't optimal. Maybe I just have weirdly-shaped ears? A quick Google search revealed that no; I'm not the only person having trouble.
It also revealed a very simple and elegant hack; replace the stock ear bits with Jabra Eargels such as those for the BT250. I happened to have a spare set of Jabra Eargels (for my backup wired headset, ironically) which did in fact fit and it presses the headset hard enough against my skin (essential so that the "jawbone" pickup microphone works) so I was able to do away with the sproingy behind-the-ear wire loop in the process. I actually shook my head hard (like a dog after a bath) and the thing stayed put without the ear loop. Added bonuses; the incoming audio is much louder since I can rotate the Eargel to align with my ear, and I can slam the thing into my ear within one ring versus time wasted fiddling with the ear loop.
Remains to be seen how it works in daily operation. I'm already missing the audio feedback tones I got with the Plantronics 645. For example when you dial a call with the P645 it gives you a tone sequence to tell you the call is connected, disconnected, etc. The Jawbone is just basically an audio conduit. You get tones for things like volume up, power on/off, Noise Shield on/off, etc but no tones for call processing status. On the other hand; I can already tell the Jawbone's range is better; which isn't hard to do given the unbearably short range of the P645. I once had the P645 go out of range on me while the phone was on my belt.