Each one of the 3 Gabriel Knight games can be considered a landmark in the history of videogaming, and videogame adventures especially. If you think about it, across the 3 games of the saga, the franchise went through the usual, old-style 2d 'painted' adventures, then onto the short but consistent phase of real-actors, and finally into the 3d era. The interesting bit is that each GK managed to be a great game even when put into a shaky context: the second game is actually one of the few 'real-actors' games worth playing, and the third is in my opinion the only, single 3d adventure to ever really shine.
Sure, the graphics are not amazing. I hear the voice dubbing is also pretty bad (I can't really comment on it, as I used to play an italian localised version of the game). And 3d in itself isn't really an adventure gamer's cup of tea, never has been and never will. But on the "adventure" side, this game has it all. A grand story which interweaves historical facts and supernatural events in the classical and loved style of Jane Jensen's previous games. A group of interesting, fun and deep side characters, each one with their own personal agenda that you can choose to uncover, even if it's just a "surplus" from actually finishing the game.
Yes, because GK:3 will actually leave you a certain degree of choice. It's not an open-ended game, but there are many moments where a careful eye can lead you to uncover new and interesting details of whatever is happening in Rennes-Le-Chateau. And if you miss those moments, you will still finish the game, but will be left hungry for more information.
The series' humour and fascinating storytelling is still strongly present in this (unfortunately) last chapter, as is Gabriel's funny but brave attitude towards mistery and danger. Just add the fact that this game includes what I honestly believe to be the best puzzle ever seen in a pc adventure... you'll love to hate Le Serpent Rouge, and you'll plain simply love Gabriel Knight 3: Blood of the Sacred, Blood of the Damned.