The fact why SiN didn't sell was that it came out the SAME DAY half-life did. Half-Life got the jackpot, while SiN was ignored by people and bashed by the critics because it had issues that were fixed by the first two patches (right now, releasing a game with some flaws is not a sin at all

)
If you haven't played it yet, I suggest you doing it (but first PLEASE upgrade the game). Five years laters, SiN STILL has some unique features in the FPS area.