I personally use a few different kinds. Either 1mm elasticated string from Michael's:
...or this stuff... which is really thick, about 1.25-1.5mm:
White Elastic Cord 144yd - Heavy
it's good for singles, doubles, multis, and other small pieces but not so good for 3Ds.
or this - a thinner version, which is probably about 1mm. Personally this is my favorite:
White Elastic Cord 100yd - Medium
^ both this kind and the kind shown in the photo are the best for 3D cuffs.
The elasticated string from Jo-ann's is horrible quality in comparison to the 3 kinds listed above :) if you've been using theirs and switch to one of these, you'll see what I mean.
I personally do NOT recommend stretch magic or any other plastic string, as I've had lots of pieces break on me or stretch out over time. If you are only planning on wearing the kandi for a few years or intend on being VERY gentle with it, it's probably fine. But if you are a long-term kandi kid and raver who frequently dances while wearing your kandi, plastic string is probably not going to last.
For a video comparison of bead and stretch and cloth-covered elastic, watch this video: http://www.youtube.com/watch?feature=player_detailpage&v=Heti6wkknhs#t=67s
As you can see, the bead and stretch stretches out almost immediately, and snaps very easily, whereas the cloth-covered string is extremely strong. That said, if you prefer Stretch Magic, Bead and Stretch, etc, and it works for you, keep using it! Just be aware of the dangers, and remember these alternatives in case you do have problems further down the line :)