syklonic: If anyone wants to avoid fully copying the MPQ files on Windows with a GOG installed D1 this script should work when run in an admin command prompt to link the original files. It assumes the script is run from the DevilutionX folder and that it was unzipped as a child of the Diablo base folder. Ex: your path to Diablo is "<path to gog games>\Diablo\ and DevilutionX is "<path to gog games>\Diablo\DevilutionX"

Script for cmd run as administrator:

mklink DIABDAT.MPQ ..\DIABDAT.MPQ
mklink hellfire.mpq ..\hellfire\hellfire.mpq
mklink hfmonk.mpq ..\hellfire\hfmonk.mpq
mklink hfmusic.mpq ..\hellfire\hfmusic.mpq
mklink hfvoice.mpq ..\hellfire\hfvoice.mpq

Anders_Jenbo: That depends i guess, if you want to run in on xbox/playstation/linux/mac/handheld/mobile then DevilutionX wins. If you want something that expands on the gameplay then Beelzebub/Techernobog wins. They both have many of the same improvements, Beelzebub/Techernobog has better lighting, DevilutionX has broader support and is closer to the original.
My recommendation would be to try both.

Ikarugamesh: I have a question, though. Is there any way to run DevilutionX alongside GOG Galaxy so it logs my playtime?