Using the Boss ML-2 Metal Core isn't hard at all as it only has some basic parameters. This is the newer version of some of the older Metal related Boss pedals, so it has a very similar make up. If you have used any type of distortion pedal I think you'll be able to understand how to use this pedal without any problems. The pedal simply has four parameters - level, low, high, and distortion. These are always the basic parameters found on overdrive and distortion pedals, so unless you are an absolute beginner I don't think you will need to study this one too hard or even look at a manual.
The sound quality of the Boss ML-2 Metal Core is sub par in my opinion. While it does have a decent metal distortion, there is really only one sound that you can get with it. Keep in mind that I'm not a huge fan of metal to begin with but tried out this pedal to see what types of tones I could get with it. I've used the pedal with a Gibson SG and a '76 Fender Twin Reverb, so I think I'm getting a pretty accurate sound from this pedal. If you are into metal and are looking to get a thick distortion sound for playing this type of music, I wouldn't pass this pedal by, but I can't say that I would recommend this pedal to anyone else based on the sound of it.
While this pedal is reasonably priced, the fact that I can really only get a single tone from the Boss ML-2 Metal Core makes it quite unappealing for me. If you are trying to get a big metal sound, this pedal might do the trick for you but I would strongly encourage those interested to look into a variety of distortion pedals as there are a lot of them out there to choose from...