I agree that the feature is kind of buried and it would be easier to find if it had a separate icon.

Now the feature is available in the "Auto-complete" effect.
There are two options:

• fill between parallel slices (you completely segment a couple of slices and from that a full segmentation is computed)
• grow from seeds (place seeds inside structures and they will be grown in 3D to partition the volume; using a cleaned up, optimized version of the fast growcut algorithm)

The auto-complete effect now also has a preview feature, so you can inspect the segmentation and make adjustments in the input before accepting the final result.

Can one modify the parameters of the "Auto-complete" effect?

Both algorithms are great in the sense that they work optimally without requiring the user to tune any parameters. The only inputs users have to give is the partial segmentation. Do you have any particular behavior that you would like to control?

Also note that in tomorrow's nightly build there will be an auto-update option. This allows real-time preview of final segmentation for cases when update is very quick (mainly for "Grow from seeds" method for relatively small regions).

No, I think I said that because I tried once and didn't like how the result looked in 3D, but I guess that's a matter of modifying the parameters of the conversion to model, which I can change in Segmentations or convert it by myself after exporting the segmentation to a label map.

Yes, 3D model generation parameters are currently not very convenient to tune (you need to go to Segmentations module, select click Update button next to Closed surface representation, choose "binary labelmap->closed surface" rule, modify smoothing factor). We'll figure out a way to make it simpler.

Interpolation between slices is available as a separate effect now.