And yeah, I called that. (Gloat.) PLEX is money; more specifically, PLEX is the best commodity in New Eden to store value with. Gevlon asserts it. Croda at marketsforisk is another investor seeing the same thing.
PLEX is money, and it's winning over ISK for the fundamental purpose of preserving value. Can PLEX go higher? Yes, we know they can; on the Chinese server PLEX sell for 2 billion ISK. There's a lot of upside here. Can they go lower? Of course. But the path to lower starts with ISK being able to better preserve value. Is CCP doing anything about that? Nothing large. Manufacturing costs are set to rise in Crius, but it seems unlikely that they will rise anywhere near the level of the ISK faucets pouring ISK into the economy. And not only that, but if manufacturing costs do become a large ISK-sink, to a large degree it won't matter, since most of those costs will be passed on to customers. Customers who, for the most part, are getting ISK from faucets via grinding. They'll grind a bit harder, opening the faucets wider, offsetting the gain in ISK moneyworthiness that would otherwise happen.
That's why I rated PLEX as a strong buy back in May, and I did indeed buy a stack of them when the next sale happened. I got in at around 670m in the PLEX sale that accompanied the release of Cronos. (Indeed, it would have been superior to simply buy as soon as I posted that article.) So I have 20 PLEX backing up my position that PLEX are better money than ISK. (I have some older ones which I guess should also count.)
Another investment I am dabbling in is Geckos. Geckos have an absolutely restricted supply -- no new ones can be created in game. Geckos are highly useful, doing much more damage than equivalent heavy drones, with the speed and tracking of medium drones. And yet as drones, they are going to get lost from time to time, both by being blown up aboard PVP ships, and accidentally left behind in missions. Also, new missioners and PVPers will find fits that use them. As such, they seem like a great investment. So I bought about a billion ISK worth. When I did it, they were at 18m ISK, then they dropped. They have mostly recovered. Small loss, but I expect a long term gain.
There's one big problem with the idea of investing in Geckos: CCP can ruin the investment at any moment by adding new Geckos to the game. Small quantities of Geckos seem probable (i.e, Christmas), but the market can absorb them in time so long as they are finite. Whether or not CCP adds Geckos into the game in some in-game form is the real question. Right now I am betting "no".
I think Geckos will be a good investment in the long run, but (unlike PLEX) don't bet your portfolio on it. Just a fraction.