I got a pair of JH16pro CIEMs back in 2010. I have *very* long ear canals. The shape of your ear canals will affect the sound of CIEMs in a major way. I have found that my closed back Shure SRH 1540's sound better to me than the JH16pro CEIMS which cost twice as much. I think that is do to my particular anatomy. However, when it comes to IEMS, you will find that using custom or universal, how they fit in your ear will have a major effect on the sound. If the pressure increases as you push a universal iem into your ear, the sound may become muffled. In the same way, if you increase the pressure in your ear canal when inserting CEIMs (which have a near perfect fit to your anatomy, and therefore will create pressure differences) you may find the need to wiggle it a bit to equalize the pressure with the outside world and correct the sound.
CIEMs are like having sound injected directly into your brain. If you want total isolation and absolute superior sound (subject to the physical constraints of your anatomy) CIEMs are the way to go. Just realize your meat wagon (ear canals) may not be optimized for the best possible sound, and therefore YMMV with CIEMs. But you can be sure the sound of high quality CIEMs will be far superior to any other universal IEM.
If you want to try CEIMs, I would recommend starting a rigorous ear cleaning routine. I use paperclips bent into the shape of a small loop on one end, which I use, after a hot shower to remove any ear wax as much as possible. If you use Q-tips, you are pushing the wax into your ear canal just past the point where the Q-tip reaches. If you don't clean your ears free from wax you push the wax into the sound holes of your CEIM. It is much more difficult to fish out a piece of wax from a CIEM than to clean your ears before inserting them. It is a lifestyle change, that should be incorporated into your daily hygiene routine. JH Audio makes a CEIM cleaning tool, which on one end has a very thin metal loop to fish wax out of your CEIM (I recommend getting a CLEAR CIEM so that you can see where the wax is) The loop sometimes will not be long enough to reach a piece of wax that has made it's way far into the sound tube. The solution is to cut the loop and make the end of the wire into the shape of a hook to reach deeply into the sound tube and remove the wax.
That is (a gross) part of the reality of owning CIEMS. But something you should know ahead of time. Get CLEAR CIEMS. CLEAN YOU EARS BEFORE you put them in, and incorporate a habit of cleaning your ears after you shower when the wax is soft, as a part of your daily routine. Carefully fish out the bits of ear wax you will miss, out of your expensive CIEMS, as necessary, using the method described. Enjoy! ;)