It might be crap from an actual standpoint, but if you try and sell adspace to real life clients its still a good way of distinguishing your site and comparing it to competitors.
To give a real answer, I consider anything under 100,000 decent, under 20,000 very good and under 2,000 amazing.
IMO Alexa widgets also increases rank a little. At least worked for me. Maybe with a boost of 20% (now I'm ranked 19K).
i think it affects only large websites , websites with big traffic i suppose take the cream of adwords
the only way alexa would affect adsense is if you were in the top 10 or similar where you show your website. then you might get some more traffic and some more clicks.
It doesn't affect AdSense IMO, but it is still a good indicator of where your site stands, and is helpful when you are looking to sell your domain/site. It would also be useful to indicate traffic while trying to sell Ad space.
Alexa ranking can be faked easily so it doesn't matter much. Just concentrate on getting targetted traffic which converts really good.
It can absolutely be faked, however, Alexa rank is used by many to determine what they are willing to pay for advertising. Some will tell you that it is easy to get into the top 100,000. This may be true, however, don't discount the fact that being there does help when selling ads directly, or through something like text link ads, adbrite, Federated media, etc.