I just bought four pairs of the fixed cubics circular needles (the smaller sizes in wood). Then I had to find a pattern to use a pair of them on (that IS the way you're supposed to do it, right--get needles, find pattern to use them on???).
Anywho, so far, I'm loving them! I can't tell that they cause "less hand fatigue" as the site claims, but they are very nice to work with. Plus, the wood ones have definitely helped even out my stitches.
I've not tried the interchangeable ones, but I imagine they're comparable to the fixed cubics. I did find a rough spot on one of my cords, but I smoothed it out with a fingernail, and the yarn is sliding right along now. I would recommend trying them. Webs may even have a try it set where you can just purchase a few of the interchangeable ones. If you don't like them, you can probably easily sell them to someone here or on eBay!