Who said this?
I'm quite sure that most people who praise Original Sin played for ~10 hours and then moved on to other things. The game was really good at leaving a nice first impression, with the pretty graphics and and interactivity.
Then you find out it has drivel for story, the combat is always the same, it's slow as molasses, quests are unexplained, broken or plain boring, and the companions might as well be made of cardboard. The worst part is - it's all. So. Excruciatingly. Generic.
I got a few more than 10 hours out of both, but there is something about Larian games that enthrall me for act 1 and then totally lose me after that. It is usually enough for me to feel I got my money's worth though, so there is that. DOS2 did a good job of fixing quite a few of the issues with DOS1, but I wouldn't recommend it if the original bothered you that much.