I bought a Garmin 800 this spring as I needed to replace my 4-5 year old wired Powertap and decided to use the Garmin as my head unit.. I have used the 800 with the cadence sensor on the same bike as the wired Powertap and they correlate very closely in terms of distance, speed, etc. with the Powertap head. There is very little discrepancy. The only software I use is Cycling Peaks.

I didn't expect to use the maps much but I am finding myself using them more and more. New routes are easy to load onto the 800 (with bike route toaster) and it is kind of fun to plan new and different routes and then following them on the screen. I also have been more inclined to just ride on unplanned and spontaneous routes and then let the Garmin lead me home.