I find this in your face street approach interesting. If you tried it in my local town (Kidderminster) you wouldn't last long, but I have seen it done in London and no-one bats an eyelid. This is leaning into a wheelchair with a short lens and flash. Not only would a Kiddy person whack you with their walking stick bystanders would almost certainly get involved.
It can only be the anonymity that large populations grant that makes this possible. That and in London generally you just don't get involved in third party interactions.
I'm not sure the sneaky blend in approach is much better, it still invades your privacy and you don't even realise. I give a cheery wave if I catch someone using me for a photo.
Mike
I've always been in two minds about street and similar snaps of people unaware (hopefully!) of the thing one is attempting to do. On the one hand, I do admit that I believe it to be an invasion of privacy, even in public, because public does not automatically mean a statement that one is open to exploitation. But, on the other hand, there is also the thrill of the act as well as the knowledge that one is not doing it in order to mock somebody or cause them harm. At least, in my case I never want to damage anyone or their reputation: I want to make an interesting image, and in my book, people are what make most images interesting.
I suppose that above all of the obvious motivation hovers the challenge to oneself to come up with something perhaps matching the abilities of past masters of the genre. Even once would be cool. Trouble is, if we consider the challengers to be photographers working decades ago, we can't do it: life and places have changed as have attitudes and awareness and the subject's knowledge that he might be used to commercial advantage of which he will know nothing, and from which he will derive no benefit; a free model, then. Such a subject probably knows nothing about the reluctance of markets to use non-model-released images of people.
As important is the fact that should you seek permission first, you have instantly changed the dynamic of both the relationship as of the result: that person will be self-conscious and try to act; most of us are not as good at it as the kid with the pistol in Klein's iconic hit which, ironically, was a set-up!
To my memory, I've only asked strangers to let me make a snap with them twice: once a lady with a dog in the basket of her bicycle, then this one, and it wasn't street, it was a mood shot:

They have little face on display and so contribute only mass, and nothing as intrusive as expression.
Rob