I was going to make a list of good and bad, but I couldn't think of anything that's good in the current state - only things that would be good if the situation were a little different. Guarantee of a skin you don't have yet WOULD be good, except there's really no other option, is there? The skins aren't tradeable or obtainable in any other way, so of course you can only get each skin once, because what else would you do with them? 400 gem price WOULD be good if the skins were tradeable on the trading post or if we had more control over which skin to unlock, whether that means straight up buy a skin or buy a random skin for a particular mount. Pure random is bad because you can't even guarantee that you'll get a skin for a mount you own. In all likelihood most players will never unlock the griffon at all due to cost or not owning the expansion, so why are they receiving griffon skins they can't use? If the random element is going to stay, there should be mount-specific boxes or a guarantee that you'll only get skins for mounts you have, as well as a cap on how many skins are in a random box. Odds of getting a skin you like are too low. In my opinion, in order to be fair, one of these two things needs to happen: Sell individual mount skins at a fixed priceORAllow skins to be bought and sold on the trading post.With the system as is I will not buy any mount skins and will actively recommend against potential players buying into the game in the worry that this is a sign of future development.
Also, at least a few nice looking mount skins should be obtainable by actually playing the game.