It probably won't get any less biased here in the forums but I got to say Gabriel Knight 1 is, in my opinion, very close to being the best adventure game ever.
First thing's first: the story is solid. You really get sucked into the mystery from the very first minutes. It's telling that the writer Jane Jensen adapted the game's story into a 400-page novel that's actually a pretty damn good book on its own (works as a long walkthrough for the game as well :D).
The puzzles are pretty clever and you need to use those brain cells a lot. There's maybe a few almost unfair ones but nothing a quick glimpse at a walkthrough wouldn't solve. However, unlike most earlier Sierra adventures, you don't need to have that walkthrough next to you at all times in order to simply survive because Gabriel Knight games don't have The Sierra Sudden Deaths, at all. It is possible to die, but it's very rare, and you'll probably know when you're in mortal danger.
However, the technical side is quite dated, in some aspects even more so than most other games of its era. The graphics are a weird mix of 320x200 and 640x480 resolutions which makes the use of the lower resolution for backgrounds slightly jarring.
The audio is not quite up to modern standards either, except for its artistic merits: the voice actors are indeed top-notch (and many of them Hollywood-proven professionals) and deliver their lines well and the music, albeit entirely using MIDI, is really beautiful and supports the gloomy atmosphere. The recording of the dialog could have been done better though; the volume levels are pretty variable and there's some static and 'pop' sounds in the speech.
The game has a pretty frightening and pressing atmosphere which I like way too much. Actually so do all Gabriel Knight games, and it's done really subtlely. The over-arcing theme of dark voodoo, some creepy thing a character said almost casually.. and you never know for sure if there's a reason to be scared or not. I don't know many games that are able to build a gloomy atmosphere like that.
Unless you hate point 'n click adventures there's no reason not to buy Gabriel Knight 1, and even if you do, I'd suggest trying.
... and here at the end I wonder why I don't just post this as an actual review. I might.
Post edited February 03, 2010 by Zat