Is it worth it if you already have a decent programmable Thermostat
I'd say forget about the payback period and just buy it because it is cool (the first time you are warm and lower the temperature manually using your iPhone while sitting on the couch is awesome!) and looks fantastic compared to the fugly units usually installed.
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If you are actively using the Honeywell and not just relying on the schedule you set then the Nest is probably comparable, the only difference being you don't have to manually change things. (AKA - set it away when you leave the house, tell it when you arrive back home, raise it when you get home un-expectantly, etc) If you have set the Honeywell, and then just sorta leave it (or just raise the temperature when you are home) then the Nest will indeed save you some energy. Or if you are like me and have a programable thermostat that was set years ago then you will definitely get your money's worth.
The Nest offers sensors for auto away, which in itself will save you over time. It doesn't appear the RTH8500D has any sensors. It's almost worth it just for that auto away feature alone. It also has a good internet set of remote apps (Android, iOS, and the web) that allows for control even when not home. This allows for remote viewing, and control over your home (so you can see if the house is getting a bit too hot or cold for some reason, or raise/lower the temp when heading home). ( shameless plug - did review of just the 2.0 remote apps @ tasel.wordpress.com/2012/04/05/nest-2-0-web-ios-an... )
For those with great programable thermostats that lack motion sensors & internet control, the Nest is still a great buy.