It probably depends on your dragons - Do you have a Diamond and/or Black Diamond/Dark Angel? Do you have enough food to feed the two necessary dragons to level 15? I had to say "no" to all these questions, so it was out of the question for me to buy Prime Eternal immediately.
Oh, but I just realized something! The sale will end January 1st (10am PST). So you could wait until Friday and maybe they'll announce something at midnight. You'll still have a couple hours to decide.