Awesome. I took advantage of the same deal, months ago. It's nice to see it's still going, and to anybody reading here who is still on the fence about buying one and has this deal available to them...well, I'll say this much: the HD 600 is the best $400 headphone I ever paid effectively a little over $280 for (with tax, plus one day shipping, since I was impatient and couldn't wait). I also would have gladly paid the full $400 for it, and when I started saving up I was fully prepared to, figuring that the gift card deal wouldn't last until I could finally buy the headphone.
Seeing the discussion here about the 5xx series, I'm glad I waited and got the HD 600 instead. A few years ago I was very close to pulling the trigger on the HD 558 (or even stretching to the HD 598, as it occasionally flirted with the ~$180 price point), and a few times since then I considered it. I always held back based on advice I received in the DT880 thread (yes, I brought up Sennheiser in there, and nearly three years later the scars are barely visible anymore
). I think Brooko specifically weighed in and recommended that if at all possible I go for the HD 600, saying that it would be a noticeable step up from the 5xx series and would be more in line with what I was looking for at the time, which was something a bit less vivid and more laid back than the DT880 but without a substantial sacrifice in raw technical ability.