I get what bowei is saying regarding sweat, etc. impacting CIEMs. While this is true, my experience is that sweat and movement adversely impact all headphone situations. One of my favorite things about using CIEMs when exercising, is that with movement, I don't have to fiddle with them and reseat them for fit/isolation. It's true that sweat build up can cause the loss of seal to some extent, but for me, it's far worse with universal in ears. This may be remedied by universals with better tips, but coming from ie800's, for instance, they were almost more trouble than they were worth for use when exercising. This may be more a fault of those iems than anything else however.
The IPX system makes this an even better use case scenario, as the connections are now waterproof, and should be trouble free. I can speak from experience, that I have used my UERR's extensively while exercising, and they have worked great. Having had the MMCX connectors, those did suffer from occasional intermittent sound situations, likely exacerbated by sweat interfering with the connections (remedied by application of contact cleaner). The IPX cable and connectors seem to address and remedy any issues exposure to sweat would introduce.