It's hard to judge any headphone in a vacuum. Usually the most useful reviews are comparative in nature. To just put on a pair of headphones and expect to be 'wowed'... That's not so easily going to happen, unless maybe you're trying out HD800's or similar. These days, even the average ear buds you get with cheap mp3 players tend to be pretty damn decent in general, and improvements coming from expensive headphones tend to be incremental more than revolutionary, especially when you consider cost. So taking that into consideration, the best way to gain as much improvement as possible is through optimization; to buy a headphone that works well with what you like to listen to.
When it comes to the DT770, and as compared to the Sennheiser HD600 or HD598, I am not immediately impressed by them in terms of how they do vocals or instruments. But I think the bass on the DT770 is excellent and I consider it a superior headphone when it comes to trance, house or other EDM. Since that's what I listen to most of the time, they are what I'm using most of the time. They are very competent headphones otherwise, as they scale well with recording quality; good recordings show a very enjoyable sound signature and decent sound stage. I quite like the way piano, violin and cellos sound on the DT770, but in most cases where instruments are prominent you would definitely prefer the HD600. Likewise, if you're looking for an open headphone that does great with guitars, rock music, or blues, you're most likely going to find more enjoyment with a Sennheiser or maybe Audio Technica model in the DT770's price class. Otherwise, the DT770 is just a solid closed headphone in a reasonable price class, where quite honestly there just isn't that much competition. This alone makes the DT770 a very popular choice, not in the least because it's very comfortable.
It just depends. My advice is that I would always be looking for a headphone that really works with what you tend to listen to. Yeah the HD650's mids are to die for, but that's not going to be a priority when you want the kind of bass a closed headphone can give you.