I've played a lot of adventure games, and this is hands down the best one I've ever done. Gabriel Knight 3 has an awesome storyline with layer after layer of detail and intrigue. The mystery just keeps unfolding as you solve more puzzles and see more and more of the secrets behind the town of Rennes-le-Chateau. Its 3D rendering and graphics look pretty great, the music is striking, and the cut scenes are really well done. The "dream sequence" movie, and the finale, just left me staring in awe. That's what you want from a game like this.
I really liked the characters, too. The interaction, especially between Gabriel and Grace, is excellent, and I really hope we'll see this continued in a Gabriel Knight 4! The only thing I wasn't amazed by was the dialog, which had its moments but was a little too sarcastic, a little too much of the time. However, I think that's no fault of Tim Curry and Charity James, etc., who did a nice job with what they were given.
As for difficulty, I wouldn't say GK3 was very hard. There are complicated puzzles, but if you're in the right place at the right time, it's pretty straightforward. I think that's to the advantage of advancing the plot, which as I said is just beautifully drawn. Gabriel Knight 3 is well worth whatever you pay for it.