Whoever pointed out that investing in, say, index funds is much better than cars was right. They pretty much always do better, they don't require insurance, repair, etc. You're not behaving rationally if you're thinking about a car as investment.
Maybe Roadsters will hold their value better than other cars, or maybe they'll be worse. Maybe they'll even go up, and there's a tiny chance that they'll do better than a stock investment would have. If that happens, great for all of us, but it *still* would have been a bad investment idea, because the chance that that will happen is so small, and getting lucky is not the same as being good.
Buy a Roadster because you like having a Roadster. They're really fun cars, and you get a lot of attention (I'm assuming that all of the pictures people take of me aren't because of my incredible looks). Buy a Roadster because you like to own a piece of history. Buy a Roadster because they're the most environmentally friendly sports car by a really long way. Buy a Roadster because you want an Elise with more acceleration.
Don't get me wrong, I love my car. Not only that, but it also turned out to be a good investment, not because the car went up in value (it didn't), but because owning an early one made it so that I could get IPO shares in Tesla, which *were* a really good investment (I mean, I got TSLA at $17!)
But if you want to make money, do something else. Then use that money to buy a Roadster for fun!