The game is great for people that really like riddles, and spending lots and lots of time to figure out what to connect with what and what next. In other words, the game is not for everyone. Not for the impatient, and not for those that are reluctant to take notes during the game.
The graphics is beautiful, but in very low resolution (compared to current typical screen res'). The way of controlling the character needs some getting used to but it's OK for this kind of game (all you see are animations, and photos (sometimes with moving parts), and you navigate by clicking the side where you want to rotate or walk, and the photo is then replaced by a new one).
The story told by the game is not very interesting, in my opinion, but it's enough to catch you.
The best part of Riven are the riddles, and I guess that's what was the authors' intention. The riddles are sometimes really complicated and sometimes rather simple. Some of them are local, you just solve them and go along, and some of them are more global, which means you need to see a lot, possibly return multiple times to the same place, to finally figure out what's your objective here and how to complete that.
I give 5 stars because the game was really entertaining, but I must say that I wouldn't be able to finish it without a small hint or two, easily found in the internet.