Yep, I think they are underappreciated for sure. But, I still think that over time, they will continue to gain in popularity, if only because they continue to get older and more unusual. In a way it's a shame that they may get more pricey, because it will make it more difficult to get bargains and deals on cars and parts. That's one reason why I have bought a couple "parts cars" to stock up on so many parts. Never know, for example, when the hatch could get dented, BUT, I have another one!
Personally, I'm not really very concerned about any "investment" aspect of owning my Aspen wagon, that would take so much of the fun out of owning it. I don't want to worry about how much it will be worth whenever, or how much (within reason) that I put into it because it is purely a toy for me, It is one thing that I deliberately DON'T want to worry about. The whole point of owning OY for me is just to have some fun. I like the fact that F's are not hugely popular, somewhat rarely seen, and are also something of a challenge to find parts for. I also like that bargains can still be found. And, it's also fun that my friends and family don't seem to ever quite "get it" with my fascination with the Volare/Aspen.