The key advantage at the time was of course travel, but as Marc says, by 79/80 the factories had caught up. The Simons were a well thought out quality product of the gold-rush aftermarket period, Steve Simons being the inventor of the USD fork and later going on to co-found RockShox, the Mountain bike fork supplier. Interesting that the equally inventive Bob Fox and supplier that other quality 'must have' period mx product, the fox air shock, similarly ended up working in the same sector - bicycle products. Like airshox, Simons forks are by no means rare, and you need not pay anything close to the prices being asked on the ads that LWC referred to. They appear on US ebay almost on a weekly basis, tho you might have to wait a little bit for the more highly prized 12 inch travel versions which may or may not be outlawed for your time-period anyways. All the best,