We picked one up for $7.99 this weekend. My advice- be sure you are crafty.

Here's how it went at our house:
1. The two 7 year old friends give up after 5 minutes because it is too hard to get the tiny rubber bands picked up with the tiny hook and the written directions are too hard for them to figure out exactly what to pull where and how to do it.
2. The 2 Moms- and later an adult cousin who steps in to bail them out -spend close to 2 hours trying to figure it out and finally accomplish the simplest bracelet. FYI- once you have it figured out, the simplest bracelet is easy. Only took about 10 minutes to make the 2nd one.
3. Friend and friend's Mom go home, then the 2 adult females left behind decide they can conquer a more complex bracelet from the directions and spend 2 hours trying to figure it out - until they finally resort to locating a how to video on the internet and basically get taught how to do it by what was probably a 9 year old. After 2 hours, the more complex bracelet is accomplished (and by more complex, I means the next easiest one to the first one we did). I doubt we could ever recreate that one without the video to guide us again.
Result- children spend NO time doing it but like the bracelets and are probably at school today wearing the bracelets and claiming they made them. Adults spend 4 hours doing it, and wind up with 3 bracelets.

Adults actually had quite a bit of fun with it- hilarious watching each other refusing to let the thing beat them.
My tips/thoughts after spending all night last night with the thing:
1. The cheap knock offs are fine....but a. They have a plastic hook instead of a metal hook, which I think makes it a little harder to get the rubber bands pulled as they have more give- and also I worried we might break it; b. The loom is basically the same, but ours did not have the hole in the posts to stick the hook down in that the more expensive Rainbow one has which makes it a little harder to get in there and get the rubber bands pulled for the more complex designs; and c. the directions with the knock offs stink-ours was very hard to follow photos with hardly any words. I think I still prefer the knock off because it was $7.99 as opposed to $29.99, but there is a bit of a quality difference. I don't know about all the knock offs, but ours did come with a ton of rubber bands and clips. We can make a ton more bracelets before we run out.
2. The actual rainbow loom website has a whole section of how-to videos for a ton of different bracelets (including all the bracelets that were with our knock off "directions" and more). These are absolutely wonderful if you can't figure out the directions that came with yours- or your knock off. We could have saved a ton of time if we had just gone to the computer to try to find a video in the first place. I think we would have made all 3 bracelets that took us 4 hours in probably 30 minutes, had we just looked for the video in the first place!
3. If you have kids that are crafty and patient, and have good hand dexterity with small things--- if you take them straight to the videos on the intenet at the rainbow loom website instead of messing with the knock off's directions, I think they will love it. Personally, while DD probably won't touch it again, I'm very tempted to pull up the videos and make some more stuff. I want to try to conquer some of the other designs.