« Not great but better than HM »

This pedal doesn't suck. I wouldn't say it is terrible but it is still a very lousy Metal distortion pedal. There are several pedal out there that do a decent metal tone for a pedal but this really isn't one of them. I do have to say that pedal is leaps and bounds over the Boss Heavy Metal pedal that was their first attempt and metal.
ML-2: Metal Core
Extreme Metal in a Compact Pedal
Features

With its extreme gain, the ML-2 is the most potent and heavy distortion pedal ever created by BOSS. It easily delivers massive distortion and gain with heavy lows for playing Nu Metal, Grind Core, Industrial, Hard Core, or any genre that requires crushing sound. The ML-2’s ferocious and biting tone will complement and cut through any blast beats your rhythm section can create.

* Ultra-heavy distortion with sharp-edged attack for crushing metal riffs
* LOW control for heavy, low frequency sound a la 7-string guitar or dropped tuning
* HIGH control for screaming, metallic tone
* Road-tough BOSS metal construction

Extreme Metal in a Compact Pedal
Specs

Nominal Input Level
-20 dBu
Input Impedance
1 M ohm
Nominal Output Level
-20 dBu
Output Impedance
1 k ohm
Recommended Load Impedance
10 k ohms or greater
Controls
Pedal switch, LEVEL knob, LOW knob, HIGH knob, DIST knob
Indicator
CHECK indicator (Serves also as battery check indicator)
Connectors
INPUT jack, OUTPUT jack, AC adaptor jack (DC 9 V)
Power Supply
DC 9 V, Dry battery 6F22 (9 V) type (carbon), Dry battery 6LR61 (9 V) type (alkaline), AC Adaptor (PSA series: optional)
Current Draw
36 mA (DC 9 V)
Expected battery life under continuous use
Carbon: 3 hours, Alkaline: 10 hours * These figures will vary depending on the actual conditions of use.
Accessories
Owner's Manual, Leaflet (USING THE UNIT SAFELY, IMPORTANT NOTES, and Information), Dry Battery 006P/9V (6LR61)
Options
AC Adaptor (PSA series)

UTILIZATION

Pretty easy pedal to master. It has four control knobs that interact withing the tonal range. The Level knob is going to control your volume output, the Low knob is going to control your low EQ frequencies, the High is going to control the high EQ frequencies, and the Dist knob is going to control the gain/distortion for the unit. The EQ knobs are a bit different than the Metal Zone in that they are more Mids focused I feel.

I am more pleased with the tonal dynamics of this pedal tremendously more than the Heavy Metal pedal they did years ago. This one isn't as thin or metallic sounding as the Heavy Metal but still sounds brittle overall which is hard to get away from with most Boss pedals.

SOUND QUALITY

Try to keep the Dist knob down or around the 12:00 position because there is a ton of gain on tap and the more you increase it the more is becomes muddy and messy. As a general rule with this pedal, keep all control knobs around the noon position. Everything else seems to go too far in one direction or another.

Try this pedal with a Les Paul and a Marshall JCM 800 and it will get a good boost in a clean type setting. The EQ boosting capabilities especially in the midrange part is great.

OVERALL OPINION

This is a pretty inexpensive pedal that will get you a decent starter metal tone. Your not going to get this pedal on any professional stage. This is for someone looking to get into the guitar and needs a cheap metal tone. For new they come in at around $90. Not going to break the bank but not going to be your final destination. If you can afford it and don't want to buy a metal amp then try out a Zvex Metal pedal. That pedal is pretty nice.