The game is repititive without a doubt. There was a clear opportunity to have made this into a deep and involving RPG game, but instead there are about 4 mission types which you play over and over and over and over again. What it did do, it did really well. The story in my view only served to give the game an ending which many might have said was much needed.
This could have been the GTA of the middle aged world, but instead got very boring. They had 90% of the stuff right, but failed on the last and most important 10%, which is sad. If I was making the sequel to the game, I would have put in a deep upgrade path weapons, armour, money even to buy better horses. Real variation in the missions is also needed. Missions that require involvement of NPCs or multiplayer play or require multiple actions in order to complete. A bit more intelligence from the guards is amiss, and ability to hide kills. Less linear game play, an assaassin guild / bounty system would be more fun.
Back to your question, is it worth buying on the PC - well for the first few hours you will have a lot of fun. If you don't get bored easily it will last longer. The scenes in the lab can just be plain skipped and serve no gameplay purpose. Try it - you might like it.