You can look at it this way. The 500 was an evolution of the 705 but didn't have the navigation capabilities. The 800 is an evolution of the 500 where they added back in navigation that is more similar to their Nuvi units than the 705. Compared to the 705 the 800 has a better mount, more bike profiles, more screens, better calorie calculations, etc. I'm not aware of any features of the 705 that the 800 doesn't have. Is there anything specific that you are concerned about?
Just curious, UMD. Are you using a Garmin now and are you getting an 800? Only asking because I respect your opinion.