I guess it depends on the relationship/ situation.

In my personal experience, it wasn't worth it. My boyfriend of 4 years went on to really hurt me during the last year we were together. I should have called it off much earlier but I was young and scared to be alone.

He was signed up for dating profiles, IM'ing with other girls, and just straight up disrespecting me. Hell, he had even offered a girl money and support when she gave him a sob story about her living situation. Just fucked. To this day, I'm pretty sure he never physically cheated on me (because he had like 0 game and would rarely leave his parents' house), but the intent was there. I almost find that worse - when someone is working TOWARDS cheating on you. Anyway, I would catch him, ream him out and then he would give me some lack luster apology - rinse and repeat. The only thing his second, third, fourth chances did were make him better at hiding all the terrible things he was doing behind my back. It turned me into a 'psycho' girlfriend. Checking his facebook, emails, text messages, everything. I was slowly going crazy with paranoia. When we finally split, I realized I hated the person I had become. I used to be so laid back and I had turned into this terrible, paranoid witch. He actually broke it off with me and it was the best thing he ever did for me. I can't express how good it felt to tell him to 'beat it' 6 months later when he asked for me back.

On the flip side, my parents dealt with infidelity issues and went to counseling and therapy for years. They read every self book under the sun on the topic and work constantly at their marriage. More than ten years after the affair came to light, they are doing better than ever. They could have just written the marriage off and divorced but they hammered through their issues and gave it one last fighting chance.