Pros: Smooth, non-fatiguing sound, comfortable, well-balanced sonic signature
Cons: Sub-bass could be deeper, treble could be slightly more airy
My first Sennheiser was the HD555, which was a real gem for its price bracket, and then when it broke, I replaced it with the HD600. I immediately exchanged it for the HD650 as I found the HD600 to sound so similar to the HD555 (just a little bit more treble mainly) that the price difference wasn't justified. The HD650 is very similar to the HD600, but with a bit more weight in the bass, and a slightly smoother sound overall. But in any case, these upgrades are all diminishing returns, as the HD555 really is excellent already.
The HD650's clamping force is strongest of the three, but it's still comfortable enough that I often forget to take them off after I was done listening to something. The velour earcups are always the most comfortable to me compared to leather, pleather, or foam.
In terms of looks, the Sennheiser are probably lagging behind all the competitors, looking kind of drab and boring, while the HD800 takes things to the opposite extreme, looking like some science-fiction head gear.
I quite like the Sennheiser sonic signaure, which many people refer to as being relaxed, or veiled. I think of it as being very smooth and non-offensive. The treble and upper mids never get fatiguing or too bright, and the overall tonal balance is very good, with nothing sticking out or recessed in any of the frequency ranges. The sub-bass rolls off around 30Hz and I wish the HD650 had more extended sub-bass and weight, since that's what it needs in order to sound like a full-range speaker system. The Audio-Technica ATH-M50, ES-10, and Denon AH-D7000 all do this very well--sounding like there's a subwoofer in the headphones. The treble of the Sennheiser is nice and smooth and articulate, but it seems to lack just a little bit of that airiness that audiophiles really love.
Overall, I tend to think of the Sennheiser HD5xx/6xx series as workhorse headphones that sound great, are very comfortable, and are never offensive. They may not win awards for looks or have that really high-end "magical" hi-fi sound, but they are tonally well-balanced and very pleasant to listen to.