There's a good chance that most guitar players who play rock are familiar with this tube, whether they know it or not. This is the custom-made company tube that drives the Marshall amps to their classic distorted sound. They've been using them for years. This is a great tube to use if that is the sound you are going for.
I am not one hundred percent sure whether or not this is the same tube that Marshall used in the late 1960s and early 1970s, when they really started to make their name. I do know that by the late 1970s, when Eddie Van Halen was using these amps, these tubes were already well in place, and they haven't really left. Having said that, I think that if you are looking for a harder edged sound, one that reflects a similar, hard rock tone, then this is seriously the best thing you can find. You might even consider putting these into another amp where they'll fit, just to get the sound, most of it is in these tubes.
I think that for the most part, this tube doesn't exactly do it for me. I think that I need something a little bit less mid-range dominant, something brighter that works more as an overdrive than an all out distortion. I tend not to rock power chords and Les Pauls as much, and the Strat-Marshall combo tends not to work for me as well as the Strat- Twin. So while I personally may not prefer them, obviously enough people do that they should warrant nothing less than a perfect score.