I had no interest in Killzone on the Playstation 2; I only played it, because I had the sequels and I needed context for the series. The game was underwhelming, but ultimately kept me entertained enough. It was still forgettable and it seemed like a rough "first draft" of a series still in its infancy.
I was sure the next two games would be infinitely better. The consensus was that Killzone 2, in particular, was one of the best games on the Playstation 3 and with Sony's backing, developer Guerilla Games would have spent a lot more resources on the series past that original attempt.
I rushed through both Killzone 2 and Killzone 3 and I've had diametrically opposed experiences with the two games.