I enjoyed the challenges very much and I do think that they helped me to improve. The only issue I had was that I was in crunch mode for most of the final challenge. I'm sure the problem was that I didn't budget my time well, due to my inexperience; I'm accustomed to working at a more leisurely pace, but pushing yourself to get faster is also a form of improvement, so it will probably be worth it in the end.
I would definitely do these sorts of challenges again; I especially liked the ones geared more toward character design.