After playing through The Witcher III a few times, I did a “lap of honour” run and played The Witcher I, II and III right after another with transferred save games. It is a nice feature sure but the impact on the playthrough of each single game is pretty negligible, since it does not change that much. Also, I liked the first part for being a solid crpg but I found that the second part often plays like a half baked prototype of the third part and playing through it felt more like a chore than a nice buildup to the great third part. If somebody wants to get into The Witcher, I would recommend skipping the first two parts and just simulate a save file if you want to influence certain prior decisions.
Guys, it really depends on the type of game.