Pros: Warm sound signature, detailed, non-fatiguing
Cons: 3-blob soundstage, clamping a little strong at first
The HD650's don't do anything really wrong.
They have a natural sound signature and slightly elevated midbass that is easy on the ear (non-fatiguing).
Most people will find that they appreciate their sound.
They are not the final word in transparency and speed, but they are advanced enough technically to allow them to be true to the source material, offering an excellent sonic window into the music.
This neutrality allows them to be used for reference purposes; as a studio headphone mixes done on them translate well to monitors.
Similarly, they sound great with all genres, and don't dominate any particular one.
For a beginner head-fi'er, HD650's paired with a <$500 SS amp and a cheap DAC might be the last headphone rig you ever need.
Inversely, the HD650 scales very well with upgrades, sounding best to these ears with a powerful tube amp.
However, If you don't like the sound of these phones on first (amped) listen, don't try change the sound signature with cable upgrades because the differences are relatively subtle.