I found Singularity to be a surprisingly engaging title, despite its warts. It's a game that seems like it was rushed to the finish line, and by now it's out of date of course, but it still holds up quite well.
The time-travel story is fun, particularly in the three different endings it provides, the weapons are entertaining despite being pretty standard, and the game play is compelling. The developers clearly didn't do as much as they could have with the time-manipulation-device (TMD) mechanics, but using the powers of the TMD well is nevertheless entertaining.
I finished the game and immediately dove into a second play-through, largely to focus on weapons I hadn't used much for more of a challenge. I'm finding it far trickier to get through the game using only the pistol and the sniper rifle. But it's fun nevertheless.
Consider it an overlooked gem that holds up well despite its age.