I've bought 12 week countdowns since I started in mid-July, and find they help me to keep going as I've already paid so feel I 'may as well' go, and also can't use not having some cash in my pocket as an excuse for not going. I also managed to time it so I got the summer offer with cookbook, autumn offer with winter cookbook and now New Year offer with free weeks etc so it's definitely been worth it for me each time!
Yes I think they are a good idea if you can afford the money up front. It does save two weeks fees if you do the 12 week one and one week if you do the 6 week one. Remember though to tell the lady/man on the pc if you want to book any holiday within your countdown period. The amount of holidays you book will extend yr countdown if you know what I mean. They should ask you anyway. Good luck