This is one of those "it depends" questions. There is no doubt many, many people are alive today who would have been dead 40 years ago before chemotherapy was as advanced as it is today.
Chemo is hellish, without question. However, many people get through it and go on to lead normal lives. It just depends on the type of cancer and how advanced it is. Early treatment is nearly always the most successful kind.
Some cancers are just not vulnerable to chemo. In these cases, oncologists have to look at other therapies to help someone.
Some people want to rail against chemo and scream that it's poison and destructive, yadda, yadda, yadda. But the truth is, it's what's available right now. A good diet and exercise may help prevent cancer and may keep a cancer patient in better shape, but it can't do everything.