If you're a newbie looking to test your reflexes, the online web version is a great stepping stone. It helps you get familiar with core mechanics like controlling the Cube, piloting the Ship, or the Gravity Ball without spending a dime.
However, if you intend to "try-hard" to conquer difficulty levels from Insane to Demon, the web version reveals an inherent weakness: input lag. In Geometry Dash, there are segments that require millisecond precision (frame-perfect). Even a slight lag or slow response from your web browser will cause your icons to "explode" instantly, easily leading to frustration and keyboard smashing.