At this point in time, I've been working with several Superlux PRA 628 MKII microphones at several venues for about 1,5 years. From what I've learned in this time, is that when I'm miking a guitar amp for a concert, I usually pick this mic. I also did a video on several cheap mics on YouTube where this microphone is also featured: get one thing straight - this microphone is not a Neumann when it comes to clarity and overall sound quality, but for its price, there's nothing that can compete with it. The most obvious rivals to this mic includes Shure SM57 and Sennheiser E609, and for sure, you can get these mics and get very decent results. But when you compare the sound of the Superlux to these two, you can make at least two distinct observations. The Shure is very picky, and you have to be careful in placing the SM57 in order to make full use of it. As the E609 is more of a direct competitor to the Superlux, they both share same kind of properties. I usually aim both mics dead center on a guitar amp, and usually the results are very similar. The Superlux sounds a bit darker than the E609, but in my mind that's a really good thing. Some amps are way too bright to be useful in a live situation, and therefore putting a darker mic on a bright amp might compensate it in a nice manner.
One thing worth mentioning also, is the tight super cardioid polar pattern. It's an excellent choice for noisy stages, as it's easy to limit the amount of other instruments bleeding into the mic. Of course you're going to get some rumble, but it's way easier to gate away stuff that is way quieter than the guitar amp itself.
The odd thing about the Superlux is the accessories that come with it. The weird gooseneck mic adapter thingy has never been of any use to me, and I think most people would prefer just to have a normal vocal mic kind of an mic stand adapter as an accessory. In most live situations, I just let the mic hang from its XLR-cable in front of the amp. That way you'll free up one stand worth of space on the stage.
All in all, if you need to mic up your (bright) guitar amp, this is the mic to go for. It's hassle-free, simple and budget friendly. Why hasn't everyone bought this mic yet?