I'll start with some honesty: I've been using cheap, old, not very good headphones for a long time. Sure, they are balanced so they gave a good representation of the spectrum but they are by no means precise... I've used this same pair for maybe 6 years.
Then my Sennheiser HD280s arrived.
Initial reactions: Wait. Wait. Wait. What? You mean I can hear the precise difference between the bass and kick drum now? You mean that the way the voice and guitar sounded muddled was just because of my headphones? What is this sense of... space? Is that... "air"?!
I didn't believe the difference would be so great but... Man I'm so glad it is. Good news though: My old tracks still sound good with the new headphones. PHEW! The headphones are a bit tight - I have a big head and I wear glasses - so I don't know if I could use them for long stretches of time until they're a bit older but otherwise I'm seriously impressed.