I bought my Hiboy Max a few weeks ago. Therefore I can review all aspects of this scooter except its durability.
The scooter is solidly built. Almost the entire scooter is made of metal, with the exception of tire, control display panel, and wire or suspension protection hoses. Even though Hiboy does not recommend multiple people riding the same scooter, it has no problem carrying 1 grown-up and 1 kid at the same time!
The 350W motor DO accelerate the scooter to 19+ mph. I actually reached such speed on a flat road. However such high speed is quite risky. I find myself mostly driving at around 8-12 mph. The power of the motor is actually needed, not for speed, but for slope climbing. For instance, when I cruise on bridge underpass, I have no difficulty keeping my 8-12 mph speed on a 15-20 degree up-hill slope!
I did not do a full range test. Usually a bar on the display (1/4 of battery capacity) can cover about 4 miles with up-hill/down-hill terrain. So the full range should be around 16 miles, which is about 2+ hours of continuous ride at my leisure speed.
The dual brakes are very handy. While they can stop the scooter very fast, you do NOT actually want to brake too hard, or else you might flip over at high speed. That is the main reason I seldom speed above 12 mph.
The 8.5" solid tire is maintenance-free and worry-free. 8.5" is the minimum size I can accept: Personally I think any tire with smaller diameter is unsafe for regular use on real world roads.
The headlight, side LED, and tail brake light are very useful in my evening or night rides. I become highly visible at night.
The only issue I had was that one of my scooters came with a broken accelerator. But Hiboy support promptly sent my a new one to put on.
Overall it is a scooter with loads of feature. I highly recommend it.