I never played this when it came out. I don't really know anything about it, and the reviews on GOG are a little biased...
So can someone fill me in? What's it like? What're it's bad points? Is it long? What's the story like? I've read that it's voiced brilliantly.

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.

Sounds pretty damn good. There's just one thing holding me back... I played a lot of Sierra adventures when I was young, and the thing I hate most about them is the solutions to puzzles don't really make any sense, and the only way to proceed is to just select every item and systematically test them on every object in the environment. Gabriel Knight doesn't have to much of that, does it? I remember Woodruff and the Schnibble... that game was a bastard, sometimes. I can handle a good amount, but not if it's just going to be a bastard to me at every screen.

The puzzles are almost all very logical and you'll know what to use when and why.
There's no using an item with another item in these games...only using them in a natural way with surroundings or with other characters.

There is an occasional stupid inventory puzzle, but 90% of the time you're talking to people and investigating in a logical manner.

I'm sold. Thanks. =]

Well, some riddles are a guide dang-it - as with every early adventure, though...
For example the way to get access to the crime scene - although one can get on this with some more or less abstract thinking.
Or getting two items from a place where you have to solve a riddle. You must read text somewhere else very carefully for that.
Both at the beginning at the game.

Yeah, that's the problem too. The few farfetched and unfair puzzles in the game are quite early, during the first 2 or 3 days maybe. That could put you off it if you don't give the game the chance to redeem itself because the rest of the game has quite nice puzzles in my opinion. It follows real-life logic quite far, or at least movie logic.
And damn if the final few hours aren't some of the most intense you can get in an adventure game.

Sorry, this won't tell you much about the game itself - but it truly is one of the best adventure games I have ever played. More than worth the $5.99 it is being charged for - if you need reviews or shots of the game you can check you tube, just keep in mind the game came out in 1993 prior to the take off of modern graphics and sound. If you want more info, check out the wiki article - [url=http://en.wikipedia.org/wiki/Gabriel_Knight:_Sins_of_the_Fathers]http://en.wikipedia.org/wiki/Gabriel_Knight:_Sins_of_the_Fathers[/url]

I'm all good for old graphics, System Shock 1 is still installed... =]
Cool, thank you for that. I'm still waiting for payday to get it because I blew all my money on wine and blue cheese, which was fantastic. But which clearly didn't last as long as Gabriel Knight will. I've learned my lesson...

Some of the puzzles are pretty terrible but that seems par for the course for these older adventure games. I just keep a FAQ handy if I get stuck for too long. It's pretty brilliant, otherwise.
Blasphemy?

Nope, puzzles especially in the earlier parts of the game are quite obnoxious. The police radio and grandpa's clock spring to mind especially. On the other hand, some of the later ones are IMO as good as any in the genre.

In my opinion the best aspect of the GK series is all the academic research you get to do over the course of each game. You'll study topics you probably never would have looked into otherwise, but you earn more than a working knowledge of those topics by the end of the game.

GK1 is well-worth the $6.
Some of the puzzles are strange, but walkthroughs are easy to come by. The infrequent frustration you might feel at a few of the puzzles is vastly outweighed by the quality of the narrative. Sierra made no better game than GK1.