I've played the witcher 1. The combat and parts of the UI (crafing especially) are not so enjoyable to be honest. The story, character building, and RPG aspects of it are great however but I am not sure if the gameplay is good enough to justify the time it takes to do a playthrough, its a long game.
The second really improves upon the combat mechanics to make it more fun, and doesn't really require knowledge of the first witcher to play.
Edit: Though at 2.50, its worth a try to see if you like it. I did not find the combat particularly fun, and I recall the crafting interface being painful.