Speaking personally, I found Riven to be far more difficult than Myst. The puzzles seem more esoteric and less intuitive than in Myst (and Myst's puzzles were already esoteric and somewhat counter-intuitive). I didn't find Riven to be as enjoyable as Myst, but if you like a puzzle-based, story-driven game where you need to exercise your imagination to its limits, you'll probably enjoy Riven.
It's my opinion that Neverwinter Nights with both expansions (As in Diamond Edition) is the best D&D based Role Playing Game ever. Not only do you get three adventures plus two bonus adventures, you also get the Toolset which you can use to create your own adventures, if you're so inclined, and the standalone server which will allow you to put your creation online, if you have a continuous broadband internet service (ADSL1 is the minimum recommended by me). Online play is free and only requires a unique set of CD keys which GOG will send you upon request once you have purchased the game. Added to this is the multitude of user-created modules and expansions (HAKs and more) available for free online. If this genre of gaming suits you, you'll have virtually endless fun. The game is old, but it's aged well.
