I only briefly touched upon this in the first 0.6 update announcement and given that progress on the new ship importer is underway, I figured I'd provide some information about what's new.
Multiple Engines and Cockpits
The ship importer now has support for both multiple engines and importing your own cockpits! For each engine trail you can toggle the flare and scale the trail however you like. All the cockpits built into the game can also be used so you don't have to make your own if you want something different.
More Settings
You are now also exposed to a lot more settings then before, almost all of the settings that have been used to configure the ingame ships! You can now also set the ship settings to any of the built in teams if you need some help with configurations.
Want to make your ship hover much more rigid? Now you can do it.
Making a hover car and don't want it to tilt when steering? You can do that too.
Improvements
You can now use a full set of transform gizmos to move, rotate and scale your ship! No more hotkeys.
The interface has also been updated to the game's new modding tools UI that was first utilized in the track creator. You now have organised dropdown menus and movable windows.
A much better scale reference has also been added, you can select from any of the ingame ships and it will display a red ghost of the ship to help you out with gettin the scale right for ingame.
Additionally workshop security has been added. If you're not the owner of a ship that has been downloaded from the workshop then you can no longer access the workshop uploader. The ship preview file can now also be chosen from anywhere you like, the image is then stored in the ship file so you can then delete the original image off your HDD if you want.
File changes
The SHP format has a new revision to support the new features such as multiple engines. This means that to utilize these features you need to upgrade the file. The game will still continue to read the old revision, but you will need to open the old ship files and re-save them back out again if you want your custom ships to support the new features.