@Simon , there are some major bugs in the FS v0.4.2 (Mac os x 10.10.5) and it is different behavior depending on the slicer engine choice. Here are my findings trying to print a 20mm x 20mm x 10mm cube as test structure.
1) Slic3r engine:
Without "pause_at_layers =" or with it but without a layer number input result, after hitting "Preview" : "Slices OK and print time shows 10m."
With "pause_at_layers = 10" or any other layer number , after hitting "Preview" the result is:
Slicer progress window shows "Getting Slicing Status" and FS hangs. I have to "Quit" FS to come out of it. It does this all the time with a layer input after "pause_at_layers =".
2) Cura engine:
With or Without "pause_at_layers =" : Slices OK BUT print time shows 39m and prints very slow. When print is terminated the dashboard window shows 60% done, although the print has completely finished! The same structure took 13m with FS v0.4.1 and dashboard was accurate. The print speed was set for both FS versions identical to 60. (But I think FS 0.4.2 does not read the speed changes and goes with the preset default 20)
Cura engine: Pause_at_Layers command is OK but the print head does not return to where it has paused. It continues from about 1 or 2 height higher ( I think it may be only one height gap and may be due to a Z increment issue).