Absolutely worth getting. For one, you get to capture either Groudon or Kyogre, Dialga or Palkia, Tornadus or Thundurus, Zekrom or Reshiram, Latios or Latias as well as Rayquaza, Deoxys, Uxie, Azelf, Mesprit, Entei, Suicine, Raikou, Cresselia, Regirock, Regice, Registeel, Regigigas, Heatran, Cobalion, Terrakion, and Virizion. You can basically get almost every single capturable legendary Pokemon in ORAS with the exception of the legendary birds, Mewtwo, Xerneas, Yveltal, and Zygarde. And with the Pokemon Bank, they can all be transferred to Sun and Moon. Plus, once you have beaten the main game, you can fly around the map and every day, mirage islands will appear in various locations and going to these locations will allow you to capture Pokemon previously unavailable in the region.
I have started a new game on my Omega Ruby and I will be attempting to capture every single legendary and every Pokemon I come across using only regular Poke Balls. Nice challenge, I think.