I am very happy with my Optimus G... Of course, I may be easy to please because my previous Android phones were entry level, an HTC Hero, and a Samsung Replenish.
When I looked at comparisons between the Optimus G and the Galaxy S3, they were close enough in capabilities that I would have been happy with either one... The fact that the Optimus G was available cheaper than the S3 was enough to get me to jump in that direction. I am happy with the decision.
Unless you have a driving need for something the LG OG doesn't offer (expansion card, or Jelly Bean, or ???) I think you would be happy.