I've got the Xbox 360 version of the Harmony which I believe is based off of the 510 model. Is the Harmony One worth it to upgrade?
Looking at the 510, the One offers control over a much larger number of devices (15 vs. 5). This might not be worth a penny to you. Just looking at the controls (as I use them in my personal set up), the One is much more comfortable from a button placement perspective. But that's a very personal opinion. Finally, you should note that a huge part of the One's functionality comes from its relatively larger LCD touchscreen display. If that cracks (as it did with mine when it dropped from an 18 inch high coffee table onto plush carpeting), the remote is completely useless. Of course, I loved the damn thing so much I got another. But that's something to keep in mind if you have kids or very curious pets.
Personally, I wouldn't upgrade unless what you have doesn't control all of your devices or it you find the button placement so inconvenient you find yourself getting carpal tunnel symptoms when using the remote.