Headphone plusses:
You hear even the most subtle details in the recording which are frequently completely inaudible with speakers.
Your always in the sweet spot no matter where you move.
You can listen to music at loud volumes at any time of day/night without too much concern about disturbing others
You can listen to any genre of music with little concern of people laughing at your music tastes...sounds silly but i do take this seriously since my tastes are somewhat off mainstream...I like the "Bangles" nuff said...
A headphone set up is smaller thus takes up far less room than a typical speaker set up. Very important consideration for some.
Can be made to be fully portable without sacrificing excellent sound quality using Headroom Cosmic or Porta Corda or DYI amps.
Headphone minuses:
Musical image is "in-head" even with crossfeed amps.
Your movement may be pretty restricted being tethered to a cord unless you have a portable unit.
Sound stage is very constricted compared to speakers which can result in musical details being lost in the compressed sound stage...i often hear things on my speakers that i don't hear on headphones so while you may hear certain details better with headphones you can also lose some details in the sound stage...
An example is in Madonna's Immaculate Collection which sounds rather bland with my headphones but explodes to life with speakers with musical details coming out of thin air from all over the listening room...
You don't get overall musical image such as feeling drum beats or bass responses, ambiance and all things considered my prefer speakers over headphones by a wide margin as being just far more "real" sounding...IMHO