Even in my early years of taking the heavy metal genre seriously and not being seasoned enough to know what's good and what's not, I never saw the appeal behind Machine Head.
Their general sound, for a lack of a better word, is shit. Imagine the bland groove metal that Pantera created in "Vulgar Display of Power" and taken to another level of atrociousness. You get bland, downtuned guitar riffs that fail at being metal riffs at all. They're not heavy, fast, consistent, or all around engaging. They're almost like slightly spiced up nu-metal with the occasional squealy guitar solo. The percussion doesn't do much to help the music, and Rob Flynn's vocals are either extremely forced clean whining or pseudo-tough guy hardcore shouts that anyone who takes music seriously would laugh at.
What really makes me loathe Machine Head is their sheer lack of integrity. During Machine Head's existence through most of the 90's, their style was akin to the groove metal sound popular at the time. When nu-metal became all the rage in the late 90's, they changed their sound to fit that template. When metalcore became the hot trend around the early 2000's, guess what, Machine Head altered their style to fit that template more. Pick a fucking sound and either stick with it or properly evolve it. It's all too clear that Flynny and his cronies only see which way the wind is blowing and adjust their sound to capitalize off the hot trend of the time.
You're much better off checking out Rob Flynn's prior band, Vio-Lence. Get Vio-Lence's "Eternal Nightmare" for some serious thrashing, heavy metal thunder.
I know that there's people that actually call Machine Head thrash metal, when they're nothing like it at all.
Finally, this band should be really ashamed of themselves for desecrating the name of a classic Deep Purple album.