How useful the MGT Mercenary Supplement is depends a lot on the military background/interest you and your players have. People with a military background as well as those with an above interest in military organisation, weapons etc. will have some serious issues with the book like
+ Lack of useful data like weight/size of the heavier weapons
+ Very vague examples of unit organisations
+ Unrealistically short ranges for some weapons
+ Misused terms and/or misunterstood concepts (i.e Frag-Canon, Energie-Mortar)
and so on. I suggest looking through the book before buying it.