While it definitely could not stand up to the phenomenon that was Half-Life (what FPS of that time could?), SiN was good entertainment in its own right. It was an excellent adaptation of the Quake II engine. I liked the level design (Levelord, booyah!) and the cheesy characters and their banter. It had cute gameplay innovations like being able to access DOS terminals and type in commands. The weapons looked cool, though they somewhat underpowered both in sound and damage. SiN was the first FPS I know to have location specific damage marks (even if it created baffling situations like a thug surviving a head shot). Sure, some of the warehouse levels were boring, but every long FPS has some filler elements.
It shot itself in the foot with a disastrous launch (in the time of 14.4kbps modems, a 20MB patch at launch just to have reasonable loading times was criminal) and didn't recover, but I suspect it has a small cult following.
On the other hand, the mission pack and the badly conceived Sin Episode(s) are rightly forgotten.