This is the speaker that comes standard in most Marshall combos and cabinets. Any 12" Marshall speaker is most likely this, unless it's a used one where the previous owner replaced it.
I am a pretty big fan of this speaker. Most people who play Marshall are also big fans, whether they know it or not. I think that the best thing about this speaker is the way that it lets you control the tone mostly from the amps controls. There is not a whole lot of bias in terms of frequency response, which is good if you need a somewhat variety of tones.
This is a double-edged sword, though. There is a chance (especially with more seasoned players) that they do want a specific bias in the frequency response. I personally think that I prefer a speaker that has a little bit more mid range and a good, clear high end that doesn't pierce. These things aren't as easy to achieve with these speakers, if at all. They really are great for the rockin' EVH tone that everybody loves (it is a great tone), but for me personally, it's not perfect.
That being said, it is a fine speaker, one that has served many a happy Marshall customer. Considering how influential that sound has been on music in general, these speakers certainly have a reputation that speaks for itself, loud and clear. These are high quality, and though they might not be perfect, they are certainly reliable.