Sorry to pour cold water, but these things are usually just a mixture of herbs, vitamins and Chinese superstition, and will do absolutely nothing for maintaining an erection. Probably the best thing to take along side viagra, and this stuff really does work on blood vessels, is gingko biloba. It helps the circulation, its good for a lot of things, like leg cramps. Its also dirt cheap. Don't take it if you are on a blood thinner like warfarin though, but apart from that its perfectly safe, buy it in any health shop and even some supermarkets.
By helping increase blood flow it can give viagra an extra little boost.