I've completed a couple of projects in my house using Speedfit including a New bathroom and moving an airing cupboard with a vented cylinder.
I've only had one leak in the year the system has been installed and that was where I had accidentally put pressure on the grip ring of a fitting but didn't have a collet clip installed.
I chose Speedfit over Hep2o because of the superseal inserts that speedfit use, meaning that there is more than the one rubber o-ring between your house and the mains water supply! :d/
However, I get the impression that speedfit is aimed more at DIY'ers and the the plumbers who are willing to use plastic piping (mine wont go near the stuff and threatens to replace it all with copper!) seem to favour Hep2o.
So does anyone have any real resons to choose one product over the other either generally or in specific situations?