Some background from IMDB.
A highly regarded neo-natal surgeon and a fellow of the American College of Surgeons, Addison is also a board certified OB-GYN with fellowships in maternal-fetal medicine and medical genetics. She also completed two years of study on cystic fibrosis. In addition, she is also one of only a handful of surgeons in the world who knows how to separate fetal blood vessels. She is increditably wealthy but it was never clearly implied if she was weathy to begin with or if she had to work for her success. Addison met her future husband, Derek Shepherd, in medical school and completed her residency under the supervision of her now good friend, Dr. Richard Webber, whom she, at one point, avoided for almost a year after he taught her a valuable lesson about not getting too close to her patients by putting her on a case in which the patient would inevitably die.
Addie and Derek became extremely successful in their respective fields and each began their own practice in New York. This strained their marriage. After she was caught cheating with Derek's best friend, Mark Sloan, he moved to Seattle to get away from her. There, he met Dr. Meredith Grey with whom he started a relationship. Addison then arrived in Seattle at Richard's behest, and confronted Derek on his affair by introducing herself to Meredith as his wife.
After a period of antagonism between her and Derek (in which he continually referred to her as "Satan"), Addison decided to stay on as head of neo-natal surgery at Seattle Grace, signing a lucrative two-year contract after she and Derek decided to make an effort to work things out, but Derek was still in love with Meredith. After finding out that her husband slept with Meredith at the hospital-sponsored prom, however, Addison (in a drunken haze) called Mark Sloan to Seattle.
She and Derek then agreed that their marriage was over and started divorce proceedings. She asked Mark to go back to New York, but he stayed, expressing his desire to continue their relationship. Addison rebuffed him.When the divorce was finalized, Addison dropped the "Shepherd" from her last name. Wanting to make a fresh start, Derek happily relinquished most of their shared properties (their Brownstone in Manhattan overlooking Central Park and a residence in The Hamptons) to her believing that she deserved it under his initial impression that she only had a one-night stand with Mark while he carried an affair with Meredith.
Later, Addison divulged the whole truth regarding her infidelity to him which led to an emotionally painful encounter in which Derek told her to get out of his life. Addison is believed to have had a good relationship with the Shepherd family, in particular Nancy Shepherd (McBitchy), but, pending Derek's revelation of the true extent of Addison/Mark's relationship, her closeness to the Shepherds is in doubt.
Since arriving at Seattle Grace, Addison has made many friendships with other characters, most notably Callie Torres and Miranda Bailey. Callie and Addison often discuss their current relationships and love lives, which has ultimately led to a very close friendship between the two. Addison is also very good friends with fellow attending, Preston Burke. She has a strained friendship with Mark Sloan and doesn't have a strong friendship with any of the interns, with the exception of Alex Karev, but that friendship is reminiscent to that of hers with Mark Sloan. She has a father-daughter bond with Richard Webber, and is often portrayed to be very close to him. A possibility of a relationship between her and Alex Karev has been hinted at, when she kissed him in Joe's bar. Even after he rejected her affections, she continued to lust after him.
She has (along with Mark) decided to abstain from sex for 60 days. If Dr. Sloan completes the challenge, she is willing to try a real relationship with him; however, it is she who fails. She engages in a sexual encounter with Alex at the hospital and Mark learns of this after witnessing them stumbling out of an on-call room. She is spared from confessing, however, when Mark lies to her by telling her he had broken their promise long before.
According to many of the show's allusions, Addison then decides to pursue Alex Karev. Her advances are once again rejected, however, hinting at the potential departure of Addison though she has expressed immense interest in being the next Chief of Surgery. Addison visited her college friend Naomi in a Private Practice is Los Angeles to get pregnant. It turns out that Addison't cervical count is 2 and she can't become a mother. Dr Pete Finch kisses her to "remind her that she's not dried up"...
Sigh. It sounds truly awful. Has anyone seen it? Thoughts?