The new tool is slicker, less frightening, then the original tool. Google said it brings more "room to continue to improve on its functionality, and over time, we expect it to replace the previous Mobile Friendly Test." The tool also is mobile friendly, so you can test sites if they are mobile friendly on your smartphone. It seems like they mainly upgraded the backend infrastructure so they can scale it as time goes.
Here is a screen shot:
Here are examples of some errors:
Some folks are reporting the tool does not work on smartphones but John Mueller said on Twitter that it is still rolling out, so be patient. I tested it and it does not seem to work on my iPhone, it just spins and spins the robot tester:
Forum discussion at Twitter.