This is coming from someone who has played almost every Lucasarts and Sierra game, but this game has the kind of gameplay and plot that surpasses ANY other game i've ever played (on par only with its sequel, Dreamfall). (I know those old games bring back nostalgia, but being older doesn't automatically make something better)
If you want a highly-immersive game that you won't want to put down, look no further. Best adventure game, period.