I'd have to agree, some got pretty boring after a while. But I did still try to complete as many as I could without losing my sanity to boredom. I'd say I completed 95%+ of the missions on the main storyline and maybe 80% on the DLC. Some of them weren't exactly worth the effort, not enough EXP and/or cash/weapons. Eventually I'll complete all the missions on both the main storyline and the DLC just to say I did it but not in the immediate future.