What she said.
Also, whether the interchangeables are more cost effective depends on what kind of knitting you do. If you use a lot of smaller needle sizes, probably not. If you use needles within your set's range, probably. If you hardly ever use more than two or three sizes, then, probably not.
OTN: Eyelet Chemise in Handmaiden Sea Silk (colorway: Midnight). Still. And a purple Donegal Tweed set of fingerless mitts, to try out my new Hiya Hiya interchangeable needles.
Latest FO: A shrug for an Anthropologie swap in beautiful Casbah sock yarn, in Cedar, a dark green semi-solid, my own pattern. Also a quick Noro Silk Garden neckwarmer for my friend Aideen, in a vine lace pattern.
My knitting blog, Another Long Yarn