I don't use the Ballistic Knives because I use tactical button layout. Combat knife just lets me use the right trigger to stab and slash. (Really wish I could use left trigger to melee with the ballistic knives.)
Concussions are good for slowing down targets, or to use against people who've spotted me at range so I can slip away. Combat axe is for people who I realise I won't be able to catch up with, or for taking out someone who's camping or with someone else.
If I'm up against a team that use a lot of equipment like claymores or shockcharges, I have another class with EMPs instead.
Honestly, I wouldn't use EC perk if it wasn't for the fact you have a really short sprint time otherwise. -_- It's the only reason it's so difficult.