When I needed a new headset recently, I ordered the Jawbone 2 (the immediate predecessor to the Prime unit... the same physical design) and regardless of all of the different earpieces and ear clips, I could not get a decent fit. Regardless of what I did, the thing would feel like it was falling out whenever I moved my head (and several times, it did fall out).
Some people swore by using some Jabra earpieces in place of the ones that came with it, but I figured that if this was supposed to be such an engineering marvel, you shouldn't have to jerry-rig it to get it to work. It went back and I ordered a Plantronics Discovery 925, and I love it. It has a new type of earpiece that provides a snug fit in your ear without cramming it into your ear canal or using an ear clip. There's no threat of it falling out at all. I got it to fit right away, without all of the screwing around that I did with the Jawbone to no avail. And the noise cancellation is very effective.
The 925 may not be as flashy as the Jawbone, but it sure does the job for me.
