Just to explain my votes...
I'm probably in the minority, but I had to go through several refits. The sound quality and isolation were improved with each fit, but the first couple of fits were poor on both sides. After my 2nd refit, I took a new impression of my left ear canal, which led to an improved but still not satisfactory fit. The right tip was still too short and both tips were popping out whenever I opened my jaw, so I had one more impression taken on both sides with my jaw opened slightly wider.
Now, I'm almost totally satisfied with the fit and isolation I get. It's still a bit tricky sometimes to get the proper fit on my right side because the tip is a bit too short and narrow. And it's a little painful to remove the tips after a few hours of listening because I have to angle them sharply up to get a proper seal. But the sound quality is heavenly: ultra-detailed and clean but powerful bass.
The lesson I learned is to get a wider-jaw impression from the start (put the bite block further back in your jaw on one side of your mouth) and, if the first set doesn't seal properly, be very specific about how you think the fit could be improved. In my case, the tips had to be lengthened and widened. I don't think it should take more than 1 refit to be satisfied if you do this.
One last thing to note: I don't find sleeping with my UM-56 to be very comfortable--they feel hard against my ears because I sleep on my side. I switch to Complys if I want to sleep while listening to my W3. Once you get used to the sound of the Um-56, however, it's hard to listen with the stock tips.
