If you are registered, you get access to the members only section, can participate in the buy & sell second hand forum and last but not least you can reserve your preferred username before someone else takes it.
I was offered a mint condition one recently, and although I already had one, mine had some small coating marks on the rear element. I always wondered if they really made any difference to image quality so bought this one and tested them against each other.
Both produced exactly the same results, so I've decided to sell the mint one as I'm not really worried about cosmetic condition, just image quality.
I doubt anything will ever exceed this lens's quality, and my remaining copy will have to be prized from my cold, dead hand.