I purchased both the Metal and the Classic Rock version of the HB Rockplug, and they compliment each other very well.
This metal version has a good variety of tones. With the gain knob, you can range from crunchy overdrive/fuzz all the way through filthy metal shredding distortion. There are virtually no cleans, but that's not why you buy this! The tone knob does a good job coloring the sound, and the volume keeps you from blowing your ears off.
The aux input makes it easy to connect to your phone/laptop in case you want to play along with a track in the same set of headphones (be aware though, the 1/8" male-to-male cable they provide is laughably short and not usable). The Rockplug comes with a USB cable for charging, which is much easier than always replacing batteries in some other brands' models.
The casing itself is plastic and not super sturdy. The on-off switch on mine is a bit funky and doesn't quite click into place, but for £11 I don't expect it to be perfect.
Absolutely worth the price! Great stuff HB!