I know that they are sharper than a normal weapon, but what is the most they can cut threw? Can they cut threw normal metals and glass? I know they are not light sabers and can cut threw a door with a feint swing, but are they sharp enough to get threw some thin walls and un-reinforced glass?

Well in technical terms a mono-edged sword should be insanely sharp; cutting through almost any material with a slight swing.
But by the rules they get +2 pen and don't count as primative. Let's look at what that means.

Now the 2 Pen means they would ignore 50% of the cover a Light wood, Armourglass or light metal would provide (according to the chart on page 199 of the MRB) and would completely ignore the protection provided by leather, furs, thick hides, chainmail and even feudal plate armour. So that's pretty **** sharp.

Now if you add the damage from a good swing, let's say a guy with strength 45 or so who rolls a 6 on the damage dice for a total of 10 damage 2 pen; that'll cut clean through light wood, armourglass, light metal AND heavy wood, flakboard and sandbags!

So by the rules a mono weapon is **** sharp; a good swing will cut through a wooden or aluminuim door, hack through a thin plaster wall or take someone's arm clean off. A 2-handed swing putting all your strength into the blow would probably be enough to hack through the side of a reinforced steel structure, take the head off a marble statue or split a grown man from head to crotch!

Well, thanks Evil. Great comic btw, I am sure this is right, but i'd like to double check. Can you add a mono edged melee attachment onto a weapon? Or is it restricted to just be a Melee attachment, also can the melee attachment be upgrade to best/good quality?

The melee attachment upgrade for ranged weapons is basically just the mounting bracket for the bayonette or whatever. The actual melee weapon you put in it can, indeed, be mono-upgraded (or even shock, chain, or power) and of whatever craft level you want to pay for. The rules just say you treat it as a spear (presumably, that means it's two-handed and does at least 1d10 of damage with a standard, common quality knife attached). Mono, exotic weapon tech, and craftsmanship would, I imagine, apply the same +dam and pen mods of the standalone weapon.

The eldar have long had Harlequins Kisses/monofiliament injectors and related technologies, so the traditional monofilament whip as what cuts through anything certainly has a home in 40k.