I don't have an interchangeable set, because I already have so many needles. But if I was starting out, and my options for buying were limited, I'd definitely invest in one of those. And then, depending on what else you'd like to knit, maybe get some circulars in any sizes that are not covered by the interchangeables. Then you'd be in a position to knit just about anything, especially if you learn one of the techniques to use two circulars for small diameters.
You could also invest in dpns if you'd be interested in circular knitting without using multiple circulars.
Since you can use yarn as stitch markers and dpns as cable needles, the only other think I'd find essential is some tapestry needles for finishing. You can use yarn for stitch holders, too, if you have a tapestry needle.