I know there are curmudgeons out there that wouldn't get caught dead with a Garmin (and that might have been me for awhile) but I absolutely love this device. I wouldn't ride without it now. I love being able to keep track of my rides, miles and elevation. The Edge 510 was a gift and upgrade from my first Garmin (the 500). This is a big jump up in performance but is also pretty significantly increased in size over the prior model. That said, I think it's worth it. I love the LiveTrack feature which allows my wife and family to follow my ride real-time on the web. If my wife wants to know where I am or when I will be home for dinner, she just pulls up my ride and checks out where I am rather than calling or texting. This also helps prevent them from worrying about me. Really, this is one of my favorite cycling accessories!
The 510 is larger but has much better gps tracking and battery life compared with the 500. Also the 510 uploads your ride automatically to the Garmin website after your ride using the bluetooth connection with your phone. The LiveTrack function also requires paring with your phone. I have only had the ride info not upload by bluetooth a few times because of a poor connection and if that happens I just cycle the 510 off/on for an upload while it boots up.
As one reviewer mentioned, the Garmin site is a bit antiquated and limited but it's slowly getting better with time. It keeps track of basic ride data and has a training calendar but is pretty lame compared to Strava. But, files can be automatically transferred from Garmin
In summary, great device. Love it! Wouldn't ride without it anymore.