It depends on 'what supplements' and in what context.
Generally, a carbohydrate drink can be beneficial for keeping you going when you begin to tire. The fitter/easier you are riding then the longer you can go before feeling the need for this. Some folk get by fine on water or squash, others like something a bit more. You can also get the various energy gel's and bars which can provide a quick-fix on the go. Different supplements get into the blood at different rates, so something like dextrose is very very fast acting, whereas your grannies fruit cake is much slower but longer lasting.
At the end of the ride, a recovery drink with a good amount of protein can help with recovery, although once again, the benefit depends on your level of fitness and how much exertion you've been putting in.
Taking it further, if you are training hard you could even start to look at some of the supplements used in weight building like Glutamine, Creatine, Ribose, Beta Alanine etc etc etc
I think it's fair to say that all or if not most of the supplements technically work, but how much you will benefit really depends on how hard your body is working in relation to your physical state.
HTH
Chris