The Sprintbok motorless treadmill is probably the best looking curved treadmill on the market. The slats and frame are made of wood and helps absorb shock and vibrations.
www.nohrd.com/gr/sprintbok/
There's also evidence that a curved treadmill burns more calories than motorized treadmill. Two studies: One shows that it burns 30% more and the other finds that it burns 44% more.
Studies on motorless treadmills:
Energy Expenditure During Sub-Maximal Running on a Non-Motorized Treadmill - March 2011The Journal of Strength and Conditioning Research
www.researchgate.net/publicat...
Comparing Calorie Expenditure and Rating of Perceived Exertion between the Curve and a Motorized Treadmill - 2014 Nicholas Robertson
Eastern Washington University
dc.ewu.edu/cgi/viewcontent.cg...