I have been using the Garmin 520 Plus for about 5 years. It's a great all around head unit. I started having trouble with it about a year ago. First there was issues with Live Track, and then it was the battery. I could hardly get 4 hour hours out of a 100% battery. Granted I do use a lot of features, including a bright screen so I can see it, the Varia Radar, heart rate monitor, cadence, and speed sensors. Still, I shouldn't need an external battery to get me through a century ride.
My awesome wife, who doesn't like not be able to track my rides because I'm often riding solo, bought me a new Edge 1030 Plus for my birthday! This thing is huge! I love it. With the 3.5" screen I can see all the data without my glasses. Thats probably favorite feature. One thing I was surprised about was the weight. Even though it double the size of the 520, it's only 4.4 oz (124 g). The second best upgrade from the 520 for me is the alert tone for the Varia Radar. It's much louder than the 520, and with the increased screen size, the visual alerts catch my eye even when I'm not looking right at it.
I'm still learning all the features of the 1030, and I'm sure I will never use half of them. For me, the most important part was battery life and usability. Easy to see, simple to navigate touch screen, and a great maps feature. For my wife, it was reliability with the live track feature so she can make sure I'm safe. They are pricy, but can you really put a price on safety. đ