I've been practicing with a DE since about the first of this year, so not fully a veteran yet. BBC, only shave my head. Takes about 15 minutes in the shower to catch most of my head. Then I finish up with the HeadBlade for total smoothness all over. The DE does get smoother 'in places' with each practice, but I don't have it down so that it smooth all over. Back of the head takes longer to get smooth.
Now, all this sounds like a lot of work, but I see it as a hybrid learning stage. The headblade is only touching up, so I don't wear down the blades as fast! that's a savings. And I am getting faster and smoother with the DE. So I'm learning.
Guess I would say it this way. I use the DE for as long as I have time ... then switch to the headblade when I have to get going or the water meter runs out!