I still have the original and am looking to buy this, but the major thing that always caused me an issue was the "ethereal potion bug" mentioned here: http://www.ataricommunity.com/forums/showthread.php?t=172354. Is this something that has been fixed in gog version?

The page http://www.ataricommunity.com/forums/showthread.php?t=172354 doesn't load... (at least for me).
It seems there's a problem with the host server www.ataricommunity.com.

Can you describe the problem?

How odd. Essentially, there is a monk in the game that you find as part of a monastery . He gives you an ethereal potion and you drink it so you can get Reubens key to open up the windmill (i think it was a windmill). An error could occur when you drank the potion and you could not progress unless you were lucky enough to get past the bug. the following is on the page I linked to but it would never work for me:

When the monk gives me the ethereal potion and my character drinks it, the game crash.
Solution 1: Make sure the character drinking the ethereal potion quaffs a potion of enchanted armour PRIOR to speaking with the monk and getting the ethereal potion.
Solution 2: Remove ALL magic orbs and wield normal weapons/armours instead.

I feel your pain, as I had the same thing when i first played. The only way I ever got past it was just luck... the one time it didn't crash, I was able to get the key and save. Looks like it's not solved then.