Hi russdog,
It really depends on who makes them and how well they make them, and the material they use. I almost can say that I prefer the 3-flange tips over sleeves in many cases. Case #1. Do you think you will eat or talk or sing with them on? If you do, you might find some discomfort and/or annoyance with them if they are made with hard materials (and most are). And it's not just these activities where the shape of your ear canal will change frequently. Hard custom sleeves do not adjust themselves to the changes in the shape of your ear canal. What happens is that you temporarily lose the seal when you open your mouth. When you close it the seal comes back (most of the time anyway, sometimes you need to reseat them). So when you open your mouth you have no bass and you can hear outside noise. Very annoying. And if you get an open mouth impression to make the sleeves, you will not get this effect when you open your mouth but you might start to feel a strong pressure against the walls of your ear canals after a while when your mouth is closed.
Custom sleeves made in hard materials can also pose a risk of damage to your earphones everytime you insert and remove them from the sleeves. And you need to do this when you change your filters and when you clean your earphones.
I use Sensaphonics custom sleeves on my ER-4. They are soft silicone, yet they still exhibit the above mentioned intermittent unseal problem in some rare occasions. Especially when eating. Another problem with them, and maybe other manufacturers don't do this, is that they place the earphones too far away from my eardrums. This affects the sound greatly. This is case #2. Frequency response will changed when the length of the sound bore is extended, and generally not for the better. With some modifications though the sound is 90% of what it use to be, but moving the earphones that far out is not a good idea and I hope the makers of your custom sleeves will not do this.
Custom sleeves are a bit more tricky to insert and remove from your ears. Depending of how difficult it is for you to insert standard tips (everyone has different ears so it varies), it might be easier to insert custom sleeves, but removal is not as simple. Instead of pulling them straight out, you have to twist them out. You also twist them in but most people require an extra push at the bottom, or an extra tugging of the ear, or some wigging, etc. Everyone has his/her own thing they do to get them in. Michael Shumacher, famous Formula One World Champ, can be seen quite often sucking on his IEMs to moisten them before he twists them into his ears. Then he strongly pushes them at the bottom to give them a final bit of pressure for a proper fit.
Basically if you do not have a problem with seal and comfort with any of the universal eartips, neither Etymotic nor I would recommend custom sleeves. They are for people who just can't find a universal tip that fits their ears properly.
