Neutral (flat, not Harman) open back with better bass extension, treble extension and soundstage than the 600 series, and the best clarity yet from the 500 series (but not 600 series level). There's some minor treble spikes (to promote soundstage) that some can perceive as higher still due to how the drivers are angled.
These would be great open backs for gaming, pretty much any music genre (including EDM-based tracks, which normally lose a bit with open backs), but doesn't have quite as natural a presentation or timbre as the 600 series (particularly the HD 600 and the 650/6XX), and the 6XX is currently only $30 more. Imaging is notably better than the 600 series as well, without being the absolute best in the price bracket.
If you're keen on music and casual gaming, get the 6XX and an amp, if you're keen on competitive gaming and will never get a decent amp because you hate your ears and like to keep chasing a new headphone every 2-3 years, maybe get this instead.
More comparisons and information in the previous deal listings. Enjoy!
"because you hate your ears"