My kids got the first one as a gift. They're like drugs. The first hit is free but you'll pay for it after that.
I don't find them objectionable in and of themselves but there is a collector's mentality built in to the whole thing and the website encourages the desire to acquire more and kids are susceptible to that. My kids always want to see the new ones when we go to a store that has them and they always ask about getting one when we're there (not in an obnoxious way, usually) but I just tell them to put them on their list (Christmas, birthday etc). Or they buy them with their own money. I try to use it as a way to teach certain lessons like delayed gratification and saving/money management.
But, if you'd rather not deal with the hassle at all, and looking back I'm not sure that I wouldn't, I'd just steer away from it.