A great puzzle game
The immersion factor is very high in this game. The superior quality of both the research and the conjecture on the part of the authors sets Riddle apart from most of the rest of the games in its genre. Although it could with some justice be described as Myst set in Ancient Egypt, in the end that is only to Riddle's credit as the natural mystery of the real-life locales in the game lends it an aura of credibility denied to more fantastical realms like Myst. On top of that, the quality of the writing, the plot, the pacing, the music, and the voice acting are all very high. (I was tempted to write off Gil at first as overdone, but he grew on me.) As for the puzzles, in my opinion they are just about right, both in difficulty and design. They definitely complement the subject matter, and never feel like artificial constructs put in the player's way merely to slow him down (although I haven't finished the game yet, so I might still be proved wrong).
As good as the graphics are, I wish they were better. It's hard not to want such exquisite views of all the Egyptana in the game to be rendered in sharper focus. Still, I hope that doesn't discourage anyone from playing. The graphics are plenty serviceable enough--even beautiful, given their limitations.
The Bottom Line
I would describe this game as a detective sim--i.e. a simulation of what it is/would be like to be a detective. If played in this spirit and taken seriously on the terms in which it presents itself, Riddle of the Sphinx is a very rewarding and interesting experience. You shouldn't miss it!