I kind of like the Vaporfly's, so much so that I actually purchased them a couple of hours ago after weighing everything up and also looking at this thread. Admittedly, I preferred the slight differences that Mayfly offered in terms of closeness to the ground, fit, and comfort - but it was not enough for me to ignore the Vaporfly, and the price isn't that bad for it these days, so I went for it. The Vaporfly is a bit bulkier, but in racing flat terms, still pretty good compared to other ones out there.
The reasoning behind it all, is not simply because I want a racing shoe, and, as I've explained I actually like the feel of training in a racer. Yes, it's less supportive and cushioned, but also less restrictive. However...
In time, could this actually lead to stronger feet and a better running style? That's my aim. Using Nike Free trainers supposedly claim that, but a racing flat is not too different so should aim to achieve mostly the same gains.
I have done quite a few 10 milers in them BR in training, infact I even raced a half marathon last August (2006), where I ran 75:46 - so as for being useful over the longer distances, and possibly your 20 miler this weekend, they should be fine enough.