Garmin Edge 510 vs. Garmin Edge 810
 

Is the $170 difference in cost really worth it? From what I can see the 810 has a micro sd slot, slightly better display and the ability to do routes.

The 810 has the ability to use map.img files to display a topographical map on screen. Also, with the correct file format can display cue sheet information and directions. It has a "get you back to start" capability as well as other useful tricks, including communicating with your favorite social media and bike-mapping and performance websites.

But if you don't want that stuff, buy the 510 if you still want ANT+, or the 200 if you just want on the ride data and post-ride mapping.

The ability to pre-load a cue-sheet and have it direct you as you go is huge. (Depending on the nature of your rides, of course.)

I was going to get a 510, but found the 810 performance and navigation package from wiggle.co.uk in England for $405, so I went for that. Received it in 5 days with no customs duty. It comes only with plugs for Europe and Great Britain for the charger, and so far I have not found any aftermarket US plugs, but any USB charger will work - I use my phone charger. Also comes with Europe map, not US, but I got the current US map for $30 on Amazon. I will use the Europe map in my auto GPS next time I go over there. All in all, for the price I thought it was a worthwhile upgrade over getting the 510 from a US vendor. That said, I haven't found the map and route feature that useful so far, since I have not ventured too far from my usual routes. I am taking my bicycle on a road trip next month and expect to depend on maps more at that time.

One thing the 510 has that the 810 lacks is the ability to use the soviet GPS satellites. That is supposed to speed up satellite acquisition, but my 810 by far outperforms all my other GPS units in getting satellite lock anyhow.

In addition, you can find locations by address, as well as destination type, which can be very handy. The 705 will do all that too, and you can find a lot of them for sale for eBay if you want to save some bucks.