I rarely, if ever, use grenades or mines. Really, I use the pulse mines/grenades to take out robots, and even that doesn't always work.
A lot of times, I find my throws bouncing past the enemy, or not doing much damage. With regard to the latter, I once threw a frag grenade pretty much dead center of a Great Khan tent (filled with three or four sleeping Great Khans). You'd think it would at least kill a Khan or two, if not more, but it barely dented their HP.
I just keep a few grenades, and sell whatever else I scavenge.
I'm only level 15 on my first play through and haven't even started on Vegas quests, but I have been scratching my head about explosives relative to FO3. Grenade launcher took 3-4 hits on feral ghouls to kill them on NORMAL. The only reason I was able to keep shooting is because I was hidden and lobbing them without VATS just for fun so the ferals couldn't find me. I'm only at about 40 explosives and haven't taken any perks so I can't really complain, but it would be nice to get credit for landing a grenade or getting a mob to run near a mine... they certainly put a lickin on the courier when he hits one. Seems like everytime I get suprised by a frag mine, I lose half my health and have at least two limbs out of commission... even with 10 DT armor on...
Generally it seems like I'm struggling to find enough enemies grouped together to make explosives worth it. Outdoors, they all tend to be spread out (think Fiends in their territory) or I can't get close enough to them while grouped without them detecting me and scrambling. Maybe when my sneak is higher and I'm indoors I'll like grenades more... I hope. I'm using it as my secondary weapon skill after guns.