My wife's Epic 4G activated on Straight Talk just fine, in their Bring-Your-Own-Device program. Only had to call tech support once or twice, but it activated the same day.
My Galaxy S3, on the other hand.... Ho Lee Shiiiiit. It passed the ESN checker just fine, everything indicated it should work. But the auto activation failed, and I had to call support. Support can't figure it out, they try to pass the buck to Sprint. They first said my ESN is stolen -- called Sprint, and it's not stolen. Then they said my Sprint account is still active and attached to the phone -- called Sprint, and the account is not active. Then they said my old Sprint phone number is not expired, it's attached to this ESN and they can't create a new phone number -- called Sprint, and they're like "it's your phone, do what you want with it, but it's not guaranteed to work on other providers" (especially if they are clueless imbeciles!). Each time, straight talk gave me a direct phone number to their support, and they said they need another 24 hours. After a few days I had enough, and demanded a refund.
I signed up on Ting's website, and in 5 minutes my phone was activated. No support calls, no nothing. Fully automated and it worked the first time. Wish I tried Ting first. I'm switching my wife's phone over when her ST minutes run out.