I have a Lance Mountain Future Primitive reissue. It's fun but the lack of concave makes it kinda hard to skate. It's definitely a board I'd bring to a sesh, skate for 30 minutes, then hop back on my regular board.
Those kinds of shapes with modern concave are a different story though. I have the Welcome Miller Crossbone and it's super fun, and the modern concave makes it basically feel like a normal deck with a short nose.
Just my 2c, I think if you're looking for something more functional that you can skate like a "regular" board go for a modern concave, otherwise, if you're going strictly for old-school vibes grab a reissue.
I totally agree with the concave thing.
I know if or when I stand on, or have a quick skate of one of those completely flat boards, I give it back fairly soon after riding it, but anything with a slightly more normal / modern concave, even some pig shaped boards with super mellow concave as a couple I have skated, they just feel so much more comfortable and easy to skate as per usual.
That said, I guess there would be people who skate the pig exactly as it was back in the day and who wouldn't change a thing, but I still am amazed at what we all did back then, or more so what those pro dudes did back then.
The well used old reissues of the Lance, Cab and McGill decks I have from swaps and trades don't get skated at all, even though sometimes I get them down and have a stand on them, but then put them back up and grab something like the Black Label Rat Trap Mountain board as a great cruiser that can still be skated like normal if needed.
I often see people buying these old reissues to build up as per the one they had when they were a kid or the one they could never get, but I don't really see them being skated a whole lot either, with so many of those boards going up for sale the next time a big bill comes up or something changes and they need to get rid of things.
As long as people are still buying them, there will definitely be a market for them, but there sure have been some amazing bargains out there from people having to sell things quickly from time to time as well.