To base judgement on alexa ranking only: it is stupid, simply because you can go in top 10k by buy every month few $100's woth of popunders who will make zero signup, but grow your alexa ranking sooo much. Look at livejasmin, with popunders they're like a facebook or google there. We also advertised with popunders at times and alexa reached top 10k, then when stopped returned low, but sales was the same really, just hosting bill increased that period
It is however true that lots people look at alexa ranking to decide if do business with you or how you are cool, so you may buy some popunderrs just to keep your alexa ranking cool to show friends. Also if alexa ranking is n/a or 3 millionth, this could be an useful indicator that the site is really dead - but should also check compete.com , google trends and so alexa just one of a number of other tools - these data all together is meaningful.