I cant speak to 800 but have the 705. Im quite happy with the 705 except it hangs up and i have to turn it off and back on during the ride. If i dont catch the freeze right away it could greatly impact data integrity of the ride. I also dont like the fact that you have to remember to hit the start button at the beginning of each ride. I sometimes forget and end upnmiles into the ride prior noticing i forgot to start tracking data. While i enjoy the grade % tracking i think the grade calculation is sometimes inaccurate.
Other than those things i have been very pleased with ease of use,battery life,easy syncing with HRM and cadence sensor. Overall im very pleased with the 705 so i can only imagine the 800 has even better technology. Make sure you get the bundle that offers the street maps. The turn by turn directions are very good.