The best part about this game is the atmosphere. It really feels like you're walking around New Orleans and New Orleans is such a unique city. Compare this game to Broken Sword where you spend the first part of it walking around Paris (which is also a unique city) and yet Broken Sword's environment could have been anywhere. Peppered throughout are historical tidbits, which actually are interesting and come in just the right amounts.
That's the best part of the game. Also quite fun is the feeling at the beginning that you're just sort of puttering about and maybe not taking your investigation too seriously -- that's a nice change of pace. You start off maybe going to find your friend with the police who wants you to meet him at the crime (but he hasn't told you where it was), or else you could go pay your grandmother a visit. Or maybe you just want to visit the park or look around a Voodoo museum. Eventually, however, all you want to do is to investigate the case. But your grandfather's mystery and trying to bed that woman in the limousine will still pop from time-to-time.
That dialogue is also good generally, and educational, and spoken by some very fine actors (including a Tim Curry who has use to the game to stretch his talent quite a bit). And the art is top notch. Perhaps the music is not to memorable.
Atmosphere is what all that amounts to and the game is exceptional with it, enough that it gets four out of five stars for just that, since there isn't too much else in the game. The plot unoriginal, being borrowed from She by H. Rider Haggard, and the "She" in this game is the least interesting character of the bunch and has about three lines of dialogue for the whole game and yet we SHOULD care about her the most after the hero (the game is guilty of a common movie foible that we should care about the female protagonist because she sleeps with the hero). Other issues include there not being a single puzzle that is taxing or memorable, that there are glitches a plenty (the most annoying that the music is too loud and has to be adjusted every time you load the game), and that every so often, even regularly in the first few chapters, you have no idea what to do next and must revisit every single location and when you find something to do you must then do it all over again to see if now anything's changed in any of the locations you've already visited. (I admit this the lazy quality I spoke highly of before is partly to blame for this. But give us some more of those errands to run about town so that we can feel that there's SOME reason to visit the drug a fifth time. Or else open up a few NEW locations -- there are just the old ones over and over game for the whole game with a vacation every now and then)
A final complaint has to do with ending, but there won't be any spoilers because it has to with so many adventure games, probably 90% of the games I've played. The climax rushes in with what amounts to a movie punctuated by pauses where you must do something, and it must be the correct or you die and must start from your last save (hopefully right before the movie), these "puzzles" are never ingenious, it's just some obvious thing your hero must do, or maybe it's one of two or three obvious things and the other two kill you. It is so obnoxious because it's not really a game, it just forces you to watch the movie sequence three to five times and cross your fingers. It's the opposite of exciting which is what I'd hope the end of a story would bring. It's hardly something to blame Gabriel Knight for since it across the board on adventure games, but somebody needs to rethink this, and until then just give us a straight movie.
Anyways perhaps I've spent more time on the problems that what is great about it. But it's hard to explain what is great when that is feeling brought on by the atmosphere. I heartily recommend it. And hope that the sequels will be just as good. :-)