Like bbeasley said, it's just neat to have the data in one easily accessible place. I have a 500 with cadence/speed sensor and HR monitor, and my use case domes down primarily to tracking my heart rate during rides and making sure I'm not overexerting myself when I don't want to be. Beyond that, the accompanying computer app is great to see what you've done on a larger scale; looking at totals over weeks/months/years, analytics for particular loops/rides and that sort of thing.

On the 500 or 510 question, I'd lean towards the 500. I got all excited when the 510 was announced, but was fairly disappointed when details and reviews came out. The touch screen isn't a selling point for me and broadcasting my rides seems like a great way to annoy friends on social media The edge has to go to the 510 on the display since it's a bit bigger IIRC...but it also isn't *as good* as the 500 in direct sunlight. As for readability, you can configure the 500 to display 1-8 data fields, the more data...the smaller everything gets. That should be well documented in reviews and photos, if it's not let me know and I'll take some photos of mine.