Very highly YMMV depending on your tenure, rate plan, and payment history. Also - double check that the $10 and $5 monthly discount stack. I was under the impression it was either the $10 credit OR $5 credit, not both. Enjoy your new phones! The S III is a really nice phone.
I am with T-Mobile for 2 yrs. I had 2 G2 phones before. The 2 credits do stack. I did verify with the rep about 3 times and I repeated the deal again and again so that if they ever reneged on the deal the tape can be pulled up. I also got his name and ID just to be sure if I need to call him again. I will post an update as soon as i get the phones.