There is still debate on that. Colonoscopies are expensive and invasive, but they are the "gold standard". A colonoscopy is every 10 years, the mail in sample is every 3 (check with your doctor to see what is right for you). The mail in sample is not as sensitive as well - I used to know someone who died of colon cancer 1 year after a negative mail in sample, a colonoscopy would have saved her life. If the mail in sample finds anything they send you to a colonoscopy anyway (but you get to the front of the waiting list) to fix the problem.

For people at low risk the mail in sample is likely good enough, and since a colonoscopy has other risks (including kill you) may even be on balance be better for those. However anyone with a higher risk should get colonoscopies. Where the line between low and high risk is though is very much up for debate.

The above is general discussion. For medical advice talk to your doctor - who will know the right questions to ask to figure out what is right for you.