I'm another happy user happy of the SRH-840. I use it with a Meridian Explorer and my laptop (running J. River 19) to play FLAC files at work. The closed-back, over-the-ear design gives very nice reduction of ambient and keeps my music from disturbing others. The sound is pretty mellow, IMO, in a good way, with good soundstage and imaging and nice frequency balance, if a little on the warm side. I listen to classical, jazz, bluegrass, and a little rock, and they all are highly enjoyable.
I haven't had much chance to use them on a fancier rig, but I'm very, very pleased at what these phones offer for the money.
The SRH-840 may not be "better" than my Sennheiser HD 650, but they are easier to listen through for hours at a time -- both in comfort (the 650 clamp my head too hard) and in a more mellow frequency response.