I went the coaching route because it keeps me focused. I ride with my coach (team title sponsor) often, so he sees my strengths and weaknesses and adjusts the workouts accordingly. I find I work harder knowing that someone will be scrutinizing my effort.
If you are disciplined, the book referenced a few posts above will be of great help with setting up a plan. I'm not disciplined and would just do group rides w/o a coach. Group rides are great and can be a great workout, but I'd only stress my stronger traits (sit in and wait for sprint) on them because I'm not disciplined.