They only increase a certain Type's moves by x1.1
Crunch = 80 Base Power (Non STAB)
Crunch = 88 Base Power (Non STAB + Blackglasses)
Crunch = 120 Base Power (STAB)
Crunch = 132 Base Power (STAB + Blackglasses)
You can see for yourself if you would like to use them or not. I like Leftovers or Stat Berries for competitive battles because I think a lot more people will find them useful compared to those other items.