Minion builds were definitely underrated. And I see a lot more minion builds than before, which is great. Before breaking out the nerf bat, though, it's best to give the metagame some time to adjust.
One of the reasons I started using minion builds was because I was facing lots of UBs and Sednas with high priests; I think minion builds are particularly effective against these two characters, both of which lack an AOE or a mass stun. I'm not sure how Ooze UBs do against minions, but minions seem to work well against spit UBs.
Some adjustments to make if you're facing minion builds:
1. Focus on armor as well as health in your items. That is, don't just stack pure health. Minion hits are mitigated by armor. A lot of the people getting torn up by minions in five seconds don't have any armor boosts at all (they go for unbreakable boots + hauberk early on). Scalemail is a worthwhile early investment.
2. Buy priests (the troop upgrade) at first opportunity. Minions, particularly spirits, will get distracted by lots of troops on the battefield.
3. Try not to stand still and tank against an Oak or Erebus using a minion build. Instead, run back a bit, move around and then re-engage; minions don't chase as well as demigods because of targetting. Furthermore, it's also harder for the demigod with the minions to repeatedly target a moving demigod in a crowd just because of all the bodies. If you sit still to fight the demigod and let the minions beat up on you, you are playing the minion swarmer's game.
4. Consider upgrading tower firepower early to get some splash damage.
5. Mass stuns work better on minions much better than on demigods. If you or your allies have frost nova or mass charm, use it.
6. Parasite Egg? I haven't used it myself yet; as I thought it was bugged.
I'm sure there's plenty of other strategies people use vs. minions that they could add.