A 29er mountain bike features larger-diameter wheels that excel at rolling over rocks, roots, and rough terrain with less effort. The bigger wheel hits obstacles at a shallower angle, so you maintain momentum instead of getting bogged down—this is called "rollover ability," and it's the 29er's biggest advantage. These bikes also hold speed better on flat and rolling terrain, making them popular for cross-country racing and long endurance rides.
The trade-off is that 29-inch wheels are slightly slower to accelerate and can feel less flickable in tight, twisty trails compared to smaller 27.5-inch wheels. However, modern frame design has largely solved fit concerns—riders as short as about 5'2" can comfortably ride a 29er today.
A 29er is a great choice if you ride fast, open trails, prioritize efficiency and speed, or spend long hours in the saddle. If you prefer playful, jumpy riding on tight technical terrain, you might lean toward 27.5" instead.