It's actually a good design, poorly executed. When new, the plastic Simplex Prestige derailleurs shifted better than almost everything else on the market at the time, but the plastic worn quickly with exposure to road grit and performance deteriorated accordingly. The all-metal Simplex LJ derialleur of the same design is one of the most sought after derailleurs on the vintage market and regularly commands high prices (often 3 figures) on eBay.