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I've just recently become the owner of a set of Denise Interchangeable needles, and I have to say I absolutely love them!
The concept is that instead of buying circular needles of varying widths AND lengths, you buy this set which contains 10 different widths of needle heads, from 3.75mm to 10mm, and several cables of varying lengths. When you need a circular needle of a certain length and width, you very easily attatch one to the other by inserting the tip of the cable into the end of the needle tip and twist, which locks it in place. Unattatching the cable from the needle tip is just as easy with another quarter-turn in the opposite direction. For those who find gripping the needles difficult, it is possible to use a bent paperclip for extra leverage.
The set also contains cable extenders, so that if you need a longer cable than you have you can attach two cables together. Cable ends are also included to turn your cables into stitch holders. These block the ends of the cables so that your stitches don't fall off. Handy when you need the needle tips for another project!
It is possible to get replacements for lost or broken parts, and the whole kit is covered by a lifetime warranty. The website associated with it also sells larger cables and different sized needles separately - those contained within the kit are what most knitters would use on most projects, but there will always be one project that requires something wildly different!
A few things I've found with them however is that when new, the needle tips can have a few rough edges, but this is solved by filing them down with a nail buffing board (the type that you use on the surface of your nail to polish it, anything rougher could change the needle size). I've also found that the cables are VERY stiff when cold, and need warming in your hands before they're flexible again, but for the majority of people this isn't a problem.
The needles themselves are made of a smooth plastic-like resin, which makes them very light in your hands, and are slightly sticky to slippy yarns, I'd say more than metal needles but not as much as bamboo or wood. The cables are made from plastic which seems pretty robust, although I'd still warm the cables up before trying the "magic loop" technique. There are two colours you can get - the standard blue colour, and pink (costs slightly more but proceeds are donated to breast cancer research).
At the price of $49.95 for the standard set, this is a bargain if you consider what you would normally pay for a similar collection of circular needles, and you can always spend the difference on yarn!
Needle Sizes: #5-15 US (3.75-10mm). Also makes circular lengths from 17in to 58in.