Chemotherapy for metastatic colorectal cancer

Of the 30,000 people in the UK who develop colorectal cancer each year, around 70-80% undergo potentially curative surgery. Even so, around a half of all patients eventually die from metastatic disease, typically within 5 years of diagnosis. Many patients who develop metastases are offered systemic chemotherapy to try to extend survival and maintain or improve quality of life. How effective is such intervention?