What the Super Metal is, in essence, is two Tube Screamers cascaded into each other. The circuitry uses the same JRC4558D op-amp chips popularized by the 9 and 10 series tube screamers, and found in earlier Ibanez distortion/overdrive pedals such as the OD808 Overdrive and TS808 Tube Screamer.
Many players looking to get crunchy overdrive from a tube amp use two Tube Screamers running into each other. This pedal saves you the trouble.
As an added bonus, you get a 3-band equalizer circuit built in as well, which really helps to shape your distorted tones. Also, like a TS, you get the ability to clean up your sound by rolling off the volume knob on your guitar.
I bought this pedal on Ebay after hearing the sound on an Ibanez DUE300 Multi-effects Unit, and immediately fell in love. The sound is creamy, and makes all your overtones and natural harmonics sing. Furthermore, I noticed that, even with single coil pickups, I got sustain for days! The Attack, Edge, and Punch knobs really make it easy to dial in the perfect shape for your signal.
My favourite thing about the SM9, though, is how it can make a tube amp sound solid-state. This may seem strange, since most people are concerned about making solid-state amps sound more like their valve-driven cousins. I love tube amps - they almost always sound better to me than transistors, but when you're playing Metal or Hard Rock, sometimes you NEED a solid-state amp just to get the right attack, response, and undecaying sustain to make it sound right. The SM9 does just that; it punches up the tubes' response to picking and hammer-ons, and tightens up everything else. (I'm not sure if Eddie used one in the 80s, but he must've had something like it...)
-Rock solid construction
-Great range of tones
-Cheaper than equivalent boutique pedals
-Very noisy with "level" knob cranked
-Minor tone drain when off