I honestly think that they're a wonderful concept that needs to be improved. I really can't help but reiterate a previously made point: that minecarts are simply so time-consuming to set up that mining is a pain no matter how you go about it. Most of my forts use a lot of wheelbarrows.
I feel like the best way this could be improved would be with some better designations, and perhaps with combining a couple of jobs. Right now, getting minecarts around requires you to fool around with stockpiles; making tracks isn't much better. It would be less realistic, but more fun (lowercase) and more game-like if miners could dig out minecart tracks the same way they dug out staircases or hallways, with no need for engravers. Likewise, you could have a special designated "dig" job that would be exactly like regular digging, except that you would be able to say "alright, now I've designated us to dig out this 20x20 area; any and all iron ore, coal or flux need to go in that minecart." Then you would link the dig designation to the minecart, tell it which ores, and tell the minecart how full it would need to be before being sent off. At the other end, it would just dump into a quantum stockpile in the magma forge area, the normal way, with a stop. Or it could link to a stockpile. Alternatively, or in addition, I think it would be a good idea to treat minecart stops like stockpiles in that one could assign them wheelbarrows. As a downside, the stops might be bigger, but as noted it's time-consuming to constantly expand minecart tracks if you're working in a large quarry. Wheelbarrows assigned to the track stop won't stop that problem per se, but they will mitigate it, because ore can be carried efficiently from further away to the minecart.
To be fair, neither of these proposals will do very much to help non-mining uses of minecarts. However, it's clear that Toady intended them as a mining tool first and foremost, and in light of that, we need to think about how to make mining with minecarts a better experience.
I'm going to put this post in Suggestions...