The other view
I am going to take an opposite point. I don't think you need a radar reflector, and here's why:
-If you sail during the day and watch out for big boats (and stay out of their way) how will a reflector help?
-As already written by a number of people, most of the big boats won't (or can't) move even if they do see you.
I sail on the Long Island Sound, near Port Jefferson. There are frequent ferries between Port Jeff and Bridgeport, CT. I see some big barges, and other large traffic. The sound is wide (100 miles) but not that far accross (less than 15 miles where I am). The ferries travel at about 15 kts and they won't change course for anyone. It's easy to see a ferry, and also real easy to stay out of the way.
Regardless of wether the ferry sees me or now, *I* have to stay out of the way.