I agree with Kristen that it totally depends on what you think you'll be knitting. I will never knit an afghan, so the super long ones are of no use to me. I do however knit tons of kids' items (hats, sweaters, etc) and I use my circs for small diameter circ knitting, So I buy lots of 16" circs, and the occasional 24" (which do come in handy for sweaters).
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KCR+ Exp+ SPM+ Steel+ Bam+ Addi++ Pl-- Wool++ Cot Lux+ Stash Scale+ Fin !Ent Int FI> Tex Lace Felt Flat Circ+ Dpn- ML+ Swatch+ KIP+ Blog+ SNB> EZ? FO+ WIP3 Gauge W+ Alt Sw