In my opinion not worth it. I had them all. At the end only HD600 and HD650 survived.
In my opinion:
HD58X: good value for Price, great enhanced deep bass, fair clear mids, a bit rolled off non-fatiguing highs. It is 150 ohms and has high sensibility, so it can be used with good portable equipment (iPhone/iPad/MacBook or good Android gear).
HD600: the best balance, natural sounding, bass is good quality, clear but a bit anemic, exceptional forward full mids, nice clear highs. Its 300 ohms+ impedance and lower sensitivity so it requires a headphone amp.
HD650/6XX: very similar to HD600, slightly increased bass/mid bass, thus a warmer sound, exceptional mids, although a bit warmer and more romantic forward mids, nice extended smooth non-fatiguing highs. Headphone amp is required with its 300 ohms impedance, although sensitivity is a bit higher than HD600.
HD660S: I don’t own these but the way see them is quite similar to HD58X, with far less bass (like HD600 but colder) and more sparkling agressive highs, same clear mids as HD58X. Overall sound therefore is thinner and colder than either HD58X or HD650/6XX. Mids are good but not as natural sounding as HD600 or HD650/6XX. As the HD58X, it’s 150 ohms impedance and higher sensitivity makes it a good choice for use with portable phones/tablet/MacBooks.
HD660S2: this is very interesting: best bass and sub bass of the group, still on the warm side as HD650/6XX with mid bass but not as much, the bass is speedier, cleaner but at same time firmer more dynamic and does extend down more than all other models except HD58X (better quality bass than HD58X though), same clear mids as 660S, but slightly warmer. Does not sound thin overall like the HD660S because of the better bass. Highs are both very natural, impactful and clear yet not agressive. Probably the best sounding overall, except for the mids which is still slightly bested by HD600 (natural open) and HD650/6XX (slightly more romantic). Its a 300 ohms impedance headphone and thus requires a headphone amp, although its sensitivity is equal to the 150 ohm HD660S.
There you have it. No bad choices overall, but each may have a preference. My preference is between HD660S2 and HD600 depending on what I am listening (jazz/pop/dance/house/EDM vs. vocals/rock/soul/classical) with special mention to HD650/6XX for its warmer romantic mids. Least preferred is 660S, with its thin bass, slightly shouty mids and a bit agressive highs. No wonder it’s the only pair in the series than I don’t own and not planning to own even though prices for 660S have gone down since the release of the far better sounding 660S2. It’s only my opinion, some may think differently.