I play engi as my main class a majority of the time and mines are definitely worth it. Two mines will kill anything except a liberator/galaxy. You can't just throw the mines like you used to be able to, they have to kind of "deploy" (kind of like the ammo has a delay). I definitely think they are worth it, and they are absolutely amazing at killing deployed sundys. My most recent favorite thing to do is re-deploy on your squad leader (every 15mins) and land ontop of a deployed sunderer and throw two mines down and clean out a base pretty goddamn quick.
Aside from not being able to run into traffic and throw them down anymore, I suggest you put two in the road that you know vehicles are going to be driving on. Mines now last through death and even through class-change, so just throw a few down and go fight somewhere else and enjoy your eventual free certs.
If you want to be a really huge dick, you can even shoot them. It brings spawn-camping to a whole new level because the explosion radius is so huge and all those fucking people sitting in spawn shooting out are just free kills, assuming you don't die before you plant the mine. The great part is, even if someone else shoots it - it gives you credit for the kills.
Be sure to use your turret like somebody else suggested, when selected and you hit B it will spawn an ammo box instead of a turret. Honestly, there's no reason to use the default engi ammo unless it's upgraded. (upgrades give it a bigger radius)