A lot of these woudln't be considered underseen. In order:

Primer (2004)
The Man from Earth (2007)
Donnie Darko (2001)
K-PAX (2001)
The Terminator (1984)
Idiocracy (2006) - hillarious
Timecrimes (2007)
Terminator 2: Judgment Day (1991)
The Butterfly Effect (2004)
Sound of My Voice (2011)
Terminator 3: Rise of the Machines (2003)

I blind bought Donnie Darko ten years ago at a flea market, and to this day it remains my greatest blind buy.

Yeah, it was a let down.

Sound of my Voice was good, but I prefered Another Earth from that directors films.

Yeah, Hellraiser iv Bloodlines was a shambles, but watchable. I found Hellraiser: Inferno surprislingly good. It is more crime/police detective type, but good imo. Have you seen it?