I've been supplementing with marine collagen peptides from Vital Proteins for close to a year now. I take about 12 grams a day, split between breakfast and dinner. I've noticed that while it's no miracle, it has reduced anorectal bleeding and irritation. This is possibly by toughening up the radiation-damaged tissues. There are different collagen sources out there (beef, pork, chicken, and fish). I chose fish because I'm wary of the way collagen is collected from pork and beef and don't want any disease risk since I'll likely be taking it for the rest of my life. I've also noticed other benefits that indicate it's helping strengthen weak connective tissue. There are genetic disorders that affect connective tissue health, so if you have one of them (Ehlers-Danlos syndrome is one example) it might be helpful. If your body's collagen is normal, you might not benefit.
It will take some time to see benefits. Allow about three months to see if anything noticeable happens.