Well, I don't really have much experience with current TOTL headphones apart from the LCD-2 but compared to HD600/650 its better in almost every aspect - tonal balance, transient response,imaging, bass quality and extension/texture of treble.
May be the Senns have a slight edge in soundstage department but that is to be expected considering we are comparing an open headphone to closed one (and even there RD's T50RP is pretty darned good for a closed headphone).