I previously reviewed this product quite negatively due to the price of £400 per pair, however the price has come down enough to warrant revisiting my review.
Although I do not own a Px pair, I have used them enough to test their full functionality, build quality, and every day usability - this year I've shot in the US, France, Ireland, and met tonnes of photogs, a few of whom had these.
Also I currently own the old RadioPoppers (the P1s), and have been shooting with them ever since shortly after they were released, when I bought mine. I was in love with the idea of preserving TTL signals in my triggers, and being able to trigger at 1/8000s (full power flash in sunlight) and these were the only units to do that.
The new Px pair are without a doubt better-built than the original, but they do essentially the same thing. Amazingly, however, my original P1s are still going strong, I shoot professionally and my P1s have outlasted one of my bodies... (I think I have a freak pair!) So there's probably something to be said about the build quality output from RadioPopper as a company.
Now these are at a reasonable price point, I highly recommend them.