Summary: The Sennheiser HD280 that I'm reviewing do not belong to me, they're presumably a couple years old (I didn't ask) and in good condition aside from a couple scratches on the earcups. I was curious to how they sounded, especially in comparison to the HA-RX900/700, ATH-M35 and M-Audio Q40 (all of which I don't presently have but definitely remember quite well). I do have Sony XB90EX, Fostex T50rp II (modded to be flat aside from a spike at around 8 - 10 khz) and MDR MA900.
Summary: I don't actually know the model year, but these phones have been around for about 10 years according to HeadRoom. I got mine about 3 years ago or so. I picked them out based on the data at HeadRoom, since there are not many good headphones sold locally. They describe them as very neutral, favorites among sound engineers. The frequency response graphs showed a very even response, with excellent sound isolation.
Summary: I've owned these cans over 2 years and I've watched them mature like fine wine sitting in the barrel. When I first bought them I was surprisingly disappointed. I was expecting a lot more but found their sound to be tinny and weak. After many many many hours of burn in these cans really warmed up and showed coloration in a nice way. I like to compare these headphones to my DT880 600ohm cans for reference of sound.
Pros: Great sound isolation, deep head vibrating BASS, nice treble, mids are slightly recessed but sound OK, great price to performance ratio
Cons: lacks much of a soundstage, can get somewhat uncomfortable after a few hours
Summary: The first of these that I have are going on 5 years old, I am also listening to them while I type this review. I am basing my opinion on this pair. I received a NIB pair about a month ago but have not listened to them yet. I really like everything about this headphone. I am a big fan of the Sennheiser sound signature though; they always seem to suite my musical tastes. I have a preference for closed cans, and this is a solid one for an entry level of better sound.
Summary: I always see people recommending the HD280 like it's some amazing price vs. quality gem in the headphone world or something, and when I borrowed a pair to compare to my HD555 a few years ago, I was surprised by how bad the HD280 sounded and how uncomfortable it was. The bass was anemic, the soundstage was congested, and the way it clamped on the head was uncomfortable. For sub-$100 headphones, you'll be much better off with the Equation RP-21.
Summary: First, let me start by saying I am a total noob. These were my first and still are my only pair of real headphones. Now that I look back, I probably should have bought the ATH-M50S considering what people have said about it. These cost somewhere around $120 when I bought them. I have had them for a few months and just got an amp (Pyle PHA-40) today (great amp, by the way). An amp is HIGHLY recommended.
Pros: Very flat frequency response, nice isolation, accurate, detailed.
Cons: Can get uncomfortable after a while, not a very exciting sound, lack of decent soundstage.
Summary: I originally got these headphones because I was looking for headphones to use both for late night listening (in an apartment complex with thin walls) and travel. So for me isolation was one of the first things I was looking for, and these headphones deliver that in spades. Unfortunately, it comes at the cost of a fairly tight fit around the head. Not enough to really be uncomfortable (I've used these headphones for hours on end), but enough to be noticeable.