I like to collect mines and C-4 and trying to think how utilize them as much as possible. I figured these scenarios worked good for me:

1. Use on the front door of the Van Graffs, so when they come rushing out they all explode
2. On the pathway before approaching Lanius the Monster of the East
3. Useful for caves that have Night Stalkers
4. Inside Camp Golf against the Rangers

Reverse pickpocket the mines onto NPCs and sell the C4
Use them in Hidden Valley to ambush BoS troops when you assault their bunker

If you're using them to ambush a group of bad guys place a string of them and disarm but don't pick up all but the last one they'd reach. The last one will chain detonate all the others so, instead of just hitting the first one or two guys to reach the mines you'll hit the ones behind as well.

theyre good if you set up any ambush lay em down attack and lead the enemy to em. c4 is good for the vault 19 quest if you have the exp skill.