Like Caduceus, Viper is one of the Snake ICE, with a very similar setup. It may be a little easy for the runner to pay through the traces if they wish, but truthfully told even for a one link runner that would be a minimum of four credits. Perhaps only two credits if running on the last click, but that is a very dangerous proposition. It is also something that some newer concept HB decks are trying to push - Run last click and face the dangers, or run early and let me rez my Bioroids at a reduction.
It has seen a lot of use in that style click denial deck, and has even seen some use in combination with Enigma. Its higher strength yet the same rez cost is balanced by the fact that it is a trace style ICE, but that higher strength keeps it relevant later into the game. Plus it means that other resources must be spent for the all too common fixed strength breakers to be used to get through it, which is a different sort of tax in itself.