Even more immersive and challenging than Myst, Riven is definitely a Masterpiece.
Gameplay: Riven plays almost exactly as Myst: the mouse is used to navigate through the scenery, which is composed by beautiful stills. When movement is needed, movies come into play. Riven's gameplay is extremely intuitive, as it's based only in pointing and clicking, and still is satisfactory. An "usual" movement type, like nowadays' FPSs, would be somehow more comfortable, but it'd sacrifice the beautiful graphics and immersion granted by the stills. For being adequate but not perfect, Riven's gameplay deserves a 9.
Graphics: The game's graphics are far away from it's time. Even today, they can be considered great - not only technologically, but artistically. The world is beautiful and unique, and all the NPCs in the game are real actors. Considering it's launch date, Riven's graphics deserve a 10.
Sounds: Perfect. I was astonished by the sounds' quality and variety. They add a lot to the immersion - you'll immensely profit with a surround speaker system. The game's sounds are basically perfect, and for that they'll have a 10.
Overall: Riven is amazing. The immersion is huge, the puzzles are hard, extremely clever and fit right into the world, not being there "just because" - there is a logical explanation and solution for everything. They're even better than Myst's, as everything connects somehow. I got hooked from the beginning, and caught myself drawing maps and taking notes of all I thought was important. It didn't last long, but, at least for a while, I was in Riven, exploring and discovering a magnificent new world... and saving it from destruction.