I use Sensaphonics Prophonic 2X-S. Just Google that to find the link. There's a number of discussions on high fidelity audio sites like ipastudio, head-fi, etc. on the merits of Westone vs. Ultimate Ears vs. Sensaphonics, the three main providers of custom IEMs. From an audio fidelity perspective, they sound pretty much similar, although you may get into an ...energetic discussion if you blaspheme with such a suggestion on some sites.
I think if you are a rider considering going this route, Sensas deserve a look for a couple of reasons:
1) I believe Sensas may be about the least protruding. In my ears, they don't protrude any farther out than the edge of my concha (the bowl-shaped area around your outer ear canal). In other words, they're pretty much flat and completely inside your ear so they won't push against your helmet.
2) Sensa 2X-S are also soft. That is, the whole driver/crossover/connector/air tube assembly is inside a soft, compliant material that is quite comfortable. I read something farther up this thread about how uncomfortable Westones are. Generally, I think any of the hard-shell customs might be uncomfortable inside a helmet. Westones also stick out quite a bit farther than Sensas.
Regarding the issue above about turning down IEMs compared to conventional cans (headphones), you actually do tend to turn down the sound pressure because of the extra isolation. Yes, they are also more efficient and put out higher sound pressure at a given drive current, but moreover they sound louder at a given sound pressure because of the extra isolation (about -25 to -30 dB). We perceive loudness of a source relative to the loudness of background noise, so isolating 'phones of any sort sound louder at a given sound pressure in a given environment compared to non-isolating headphones at the same sound pressure in the same environment.
Unfortunately, hearing damage doesn't occur as a function of relative sound pressure, but rather on an absolute basis vs. time. So there is merit to being able to perceive high sound volume at a low absolute sound pressure.
On the down side, you don't put your headphones on simply by putting your helmet on with IEMs compared to helmet-mounted speakers. It is really no more of an inconvenience than putting in earplugs. But if you tend to be really pressed for time and don't want to fiddle with something else before riding away, that might steer you toward some in-helmet solution.
What? No "rock-on" smiley?
$0.02