I got the Polar s720i and got it working. It's an incredibly complicated watch that is hard to operate. I've been using the Campagnolo Ergobrain with a Polar heart rate monitor with good success, but this means just one device (so it is lighter) and also includes power.
Main disadvantage is there are an incredible number of menus to understand. Now I'm off to get the Power Output meter. Here's where:
* "Colorado Cyclist":http://coloradocyclist.com. They have them for $315 plus shipping. This seems to be the standard price. * Polar Heart Rate Monitors. Joseph French Marketing has these for$315 including shipping, so a slight discount. I've never ordered from them before though.
* "Trisports":http://trisports.com/polarpowerkit.html. They have for $297 plus$10 shipping, but I've never used them before.