Been using a Polar Power sensor since last fall, and it's worked fine for my needs. I already had a s720i HRM (for about a year prior), and have been downloading my ride data to the Polar Software, so adding the Power Sensor was a reasonably priced choice. Now my excercises on my road bike are showing wattage in addition to speed/distance, cadence, altitude. It's been pretty cool to see my power output for a sprint, versus a hill and for my 5 different roller interval programs, it's a great measure of actual performance versus my perception of how hard the effort was. I don't know how accurate the Polar Power data is in comparison to the other systems out there. I'm not really comparing my power data to anyone else, so as long as the output wattage measured is consistent for my use, I'm happy.
Another advantage about using the Polar based system, is that I can switch between roadbike's Bike 1 setting with the Power Sensor, and my cyclocross' Bike 2 setting, with basic wireless speed and cadence sensors. I can download excercise files for either bike, which has been great for tracking in the training diary.
It took about an hour to set-up and tweak the Polar Power sensor, and a few rides to get familiar with using it. Now, I'm doing a lot of my solo training rides watching the power data and learning the relationship between my perceived efforts, actual heart rate, and power output.
Like coolhand, I'm a gadget freak. If you're comfortable with setting up and working with digital devices like syncing a PDA phones to your PC or Outlook, the set-up on the Polar Power sensor should be comparably easy.