I just stumbled upon here while searching for something totally different, but I’ll throw in my two cents on this topic (trust me, i’m an expert):

CFA is most useful for analysts (e.g., buy side and sell side analysts).  As for traders – it depends on the type of trader. It wouldn’t be as useful for a trader of a purely quantitative strategy (HFT/stat arb / quant equity); yet it may be useful for a relative value arb trader hoping to manage his own fund. The CFA designation is important mostly to outsiders (investors, clients, etc.), although it does help the general promotion potential of anyone in the industry without a graduate degree.  Whether you seek the designation or not, however, I would strongly suggest learning most of the material anyway – at least to the equivalent of level 1 (or level 2 if you trade derivatives or fixed income). If you don’t, you’ll be at a disadvantage (as a trader or a quant).  Just for reference, the depth of material in level 1 is roughly equivalent to what is covered in an undergrad Finance program or an MBA with a Finance concentration; level 2 coverage is comparable to a MSF (MS in Finance); and level 3 is a broad coverage of portfolio management.
I think there may be old exams floating around somewhere – take one and see how you do.