I have Straight Talk, DH has ATT (his company pays for his iPhone).
I have a smart phone, but not an iPhone. Anywhere DH has service, I have service and vice versa. I haven't noticed anything other than the phone that I currently use is a POS -- freezes a few times per week and has to have the battery removed to restart. I digress.
The coverage is fine, I don't know how else to describe it. I can't drive through the Sierras and hold a conversation on my phone, but neither can DH. So... IMO, it's no better or worse than his ATT coverage.