This is one of the very first games I actually finished when I played it on the Amiga. The atmosphere is great and the animations and graphics were fantastic at the time (astoundingly, I feel they somehow still hold up pretty well)
As others have said, it's *VERY* linear. The developer wants to tell a story, and you're not supposed to stray off the path. It's still fun, but at times it can get very frustrating.
The major issue though is paradoxically linked to the enhancements in GOG's version : the collision detection works nicely and is rather forgiving at lower resolutions, but if you try to play at some very high ones (1440x1050, or 1680x1050 were the two I tried) jumps have to be pixel-precise. The solution, of course, is to play at lower levels.
*SPOILER AHEAD*
This is particularly noticeable in the water cavern level, where you have to do 4 jumps while running away from the flood. I tried and tried, and it was sheer impossible to achieve. I lowered the resolution to 800x600 and I was able to get pretty lazy about the jumps. A few pixel more or less didn't matter at all and I was able to finish the level. So lower the res if you find that you keep failing in run&jump parts, and you will probably enjoy the game way more.