I started with the Garmin 510 and was happy with that for a year or so and no real complaints. Syncing it to your phone is great, never had to connect it to the computer which was nice and it offers a fair amount of features compared to the 810 or 1000 (minus navigation of course). The biggest downside IMO for the 510 is the screen... It's sometimes hard to get it to work good because it's not sensitive at all.
I upgraded to a Garmin 1000 a few months ago, mainly because I wanted to have navigation, a bigger and more reactive screen, and I thought the added connected features were a cool idea too. So far I really like it. I've used the navigation several times, both I routes I've made and routes that the Garmin came up with (round trip routing-very cool feature). It's usually very good but once in awhile it does do some confusing things but those are rare. Round trip routing (available on a couple other Garmins too) is a nice feature. Put in a starting point and how many miles you want to ride and it'll make 3 routes near that mileage and show elevation for them too. The screen is SOOO much better than my 510, much more reactive, easier to use and bigger.