I second what Julie B said about sticking with RKC certified instruction. I haven't seen Delaine Ross' DVD but I have seen some of her YouTube videos and she really knows what she's talking about. Check out http://www.dragondoor.com for all sorts of information. The website is fairly man-centric but if you ignore that the information you need is there. There are articles, good DVDs, a link for finding a RKC certified instructor near you. Look on YouTube for "kettlebell RKC" or Delaine Ross.
I have been using kettlebells for a couple of years now and they are the best, quickest all over workout I've ever done. I am stronger and in better shape than I've been in years. If only I could conquer the late night snacking I'd be fabulous, but I'm still working on that part.
Good form is super important. I haven't seen the Jillian Michaels DVD but when it first came out there was a clip from it on YouTube and it showed her demonstrating a move with truly frightening form. She is not RKC certified and while she talks a good game on TV I really believe she doesn't know diddly about how to properly use a kettlebell. That's what leads to injury. I would not recommend her or anything called kettlenetics.
ETA - Yes, you can do all the basic exercises with just 1 kettlebell. I started with a 20lb one from Dicks. As I got stronger I also bought a 30lb one. I've contemplated getting another heavier size but those do get much more expensive and I haven't done it yet. When you're shopping, look for one that has a smooth but not slick handle. You want to be able to grip it well but not have any lumps that will cause blisters on your hand.