I've used minecarts a bit, yeah. Wheelbarrows have been exceptionally helpful, especially with stone/metal. Minecarts... well, they took a long time to set up.
For example, my first minecart route was from the Magma Sea to my main fortress complex - about 150 floors up. Setting it up took forever, mostly due to ramp-based shenanigans and carving mistakes ("user error"). It made it so that I could just have my smiths and smelters pump out metal objects at the Magma Sea, then someone would guide a cart full of those objects further up to the fortress proper, where they could be taken to other stockpiles or fitting uses. Despite all its problems, the main bonus of this was that I never had to build a magma pump stack. Rather than pump all that magma up 150 floors in a consistently aggravating process, I could just set a bunch of plebs to carve tracks, then assign a few vehicles and stockpiles. It was SO much easier. Powering this particular track wouldn't be as easy or as necessary, but it worked and worked well.
Then I started messing with other minecart tracks - one from my industrial complex to the trade depot, for instance. Worked like a dream! Then I made a circuit of it and powered it, and suddenly it wasn't such a big deal to supply the caravans with my useless junk valuable trade goods.
But then I tried to make a track to refill my catapult ammunition stockpile (Siege Weapon training ground). I had a raised bridge that faced the siege entrance through some fortifications, so my practicing catapult-users could just keep firing at will when I was besieged. When raised, the bridge was over a channel, so the fired rocks would drop into a stockpile below. The idea was that a minecart would be filled from this stockpile, get pushed back to just under the catapults, and they'd load the catapults from the resulting stockpile. Powered rollers would send the cart on its way as necessary. What actually happened was that the cart full of stones got pushed to the refill properly, but then it was carried back to the spent ammunition instead of sent around the track. :/ Still haven't figured out what I did wrong there.
Smaller cart tracks could be useful for any major construction project that requires moving a lot of stone or blocks, such as clear-mining your flux stone, or if you come upon magnetite deposits. But mostly... I've found that it can really clear up a lot of your haulers for use in industry or military pursuits. The old "1/3 population=Haulers" rule no longer need apply.