This is truly one of the classic adventure games. I can't remember where I first saw the demo for this, but it immediately prompted me to buy the game (and this was back in the era when it came on floppy disks). Then I bought it again when it was re-released on CD. And I've played it several more times now through ScummVM. I have no doubt that I'll enjoy the GOG release as well.
The game is rife with dark humour (and an insulting sidekick). All of the characters in the game have their own little quirks, and the dialogue is brilliant. And while some of the events leading up to the finale are fairly predictable, the game throws enough curveballs into the works to keep things interesting.
And since it's available for free, there's simply no excuse not to download and play it. What are you waiting for?