I thought that was a great movie. But then again, I like dark movies & movies that deal with the effects of 'time travel' (BTTF, the new Flash TV show, etc.).
The concept is based on chaos theory. Basically the idea that a butterfly can flap its wings in California and, through a domino effect, eventually cause a hurricane in Florida. That every action affects every other action & multiplies over time with unforeseen consequences:
https://en.wikipedia.org/wiki/Butterfly_effect
That's the point of the movie...he goes back to change stuff, but ends up making it worse because he can't control all of the variables & outcomes.
Also, this was the first show I had ever seen Ashton Kutcher in. I was pretty shocked to find out that he was funny, like in the 70's show. Same with Kurt Russell...first thing I ever saw him in was Stargate, as a super-serious military guy, then I saw Overboard and was like whaaaaaaat the heck