If you look past the ugliness of having been made in 1967 (awkward direction and acting), it actually is interesting. This is partly with the help of the lead, Lee Marvin (not exactly a good performance, but an interesting one nevertheless), and partly with the help of his character, due to his relentless pursuit for the money taken from him. The flashbacks are badly made, complete with horrible editing. The scene where his gal unleashes violence against him is memorable.
I was led to this after finding out it's based on the same material as the later Payback, a far superior take.