Interchangeable circular needles are great. But one things catches lots of people new to them out: cable lengths!
Interchangeable needles are like circular needles, except the tips can be separated from the cable. This means you can change the length of the circular needle with a new cable, or the size by swapping out tips. You need to buy the tips and cables separately, but after a while you can get a huge range of sizes and lengths with only a few different items.
The cables for interchangeables are listed by the length of circular needle they will make. This means that they are not the length they say they are! A 60cm cable might actually be around 35cm long. But, when you attach two tips it'll then measure approx 60cm from tip-to-tip.
The rule is, if your project calls for a 60cm circular, buy a 60cm cable - don't try and do maths working out how long the cable should be! And, if you're trying to match an existing circular needle, measure from the end of one tip to the end of the other.
We'll post more about the various choices of interchangeables soon, but one key thing to keep in mind: brands are not compatible with each other. Addi tips, need an Addi cord. KnitPro tips need a KnitPro cable. And HiyaHiya tips need a HiyaHiya cable (of the correct connector size - as I said, more on that later!).
Hope this helps, and get in touch if you need any more information!