Rollercoasters can be quite fun to ride, though they mainly depend on their design to determine whether they’re good or not. You also have to keep the game mechanics in mind when you design your rollercoaster.
There aren’t really any bad or good ways of building a rollercoaster, though some things will work better than others. The main thing about rollercoasters in minecraft is that they work best when the person riding it, is playing in first person view.
First person view makes sure you’re always facing the front of the minecart, so you’ll always see whatever the tracks are leading you towards. However, first person view does limit your view quite a lot, which means you might not always be able to see the tracks.
This is something you will have to keep in mind when you design your rollercoaster, which means it’s best to constantly test it before you move on to the next section of your rollercoaster.
A downward slope will look and feel more exciting if you’re going through a tunnel, perhaps with lava flowing on the sides or above the tracks.
Making the minecart fall from great heights is a decent way of getting momentum and adding some thrill to the ride, however, people won’t be able to look downwards unless they’re in third person view. To make this fall more exciting, make them fall through rings, tunnels or perhaps between floating islands.
Another fun trick is to make the player go backwards by making the minecart land on a slope. The minecart will be pulled down due to gravity, so the player will be facing the other way (if you made the minecart land right). This can lead to some fun effects or perhaps a nice ending to the ride, by making the player look at the complete ride he has just been part of.
Try to avoid fast zig-zag tracks, they’re usually just annoying and leave the player without enough time to look at the surroundings. This could work if you make the tunnel through which this track is going out of many different coloured wools, though it will usually still be annoying to the rider.
The exact design of your rollercoaster usually doesn’t matter, as long as it looks good. You could make it look like real life (wooden) rollercoasters by making everything out of fences and wooden planks, or you could make a space themed rollercoaster in which the rider rides from planet to planet.
Players will usually look more at the surroundings than at the tracks, so it’s best to spend more time on the design and the surroundings of your track than on the turns, falls and other parts of your track. But remember to test your track a lot, it can be quite annoying to find out part of a track doesn’t work after you’ve already built the other parts.