I did it for the first time last year, thought it was pretty fun~ as others have said I think it really depends on how much is on your plate outside of the challenge and how seriously you take it. If you have tons of other stuff going on, it's not really worth. But if you have time for it and approach it casually I think it can be fun
I do admit to falling into the pitfall that @c0ldw1nter theorized about, where a fair portion of the images I drew last year were either quick sketches or just mid-to-low quality. Some days I just didn't have time to do something nice lol. Completed all 31 days, but some pictures I completed over 2 days or were just "throwaway doodles".
Even so, going to tackle it again this year! Ballpoint pen is actually my favorite doodling medium anyways xD