it isn't really that bad, it just isn't really very good, either. the general rap is that it attracts and holds more grit and debris than other lubricants. because it is in aerosol form it has solvent carriers mixed in with it, and if you spray some onto a gunked-up chain the solvents will dissolve and flush away some of the gunk and you will notice a real improvement in performance, so you will be tempted to think "hey this is a good lube." but it would be better to clean the chain first, and then apply something like triflow, ich is what i use most of the year.

last winter, commuting through snow and slush and grime, i used a lot of wd-40 because it does a good job of repelling water. i would just spray it on every day (sometimes twice) and then swap the chain every week or so (i kept two chains, one on the bike, the other getting a nice slow cleaning). i also like it because the smell reminds me of the old days when nobody knew any better, and a nuovo record derailleur sold for $35...