AT&T service is good - my boyfriend has it and he's never had an issue. I've got Sprint and it's fine as well.
Stillwater is not a bad town to live in. I feel like it lacks some culture, but it's usually pretty easy to entertain yourself around here. And I love the bars - Finnegan's and Stonewall especially. Granted, I've never been to bars anywhere else, but these are really chill and pretty cheap comparatively, from what I've heard.
I will say that after 4 years in Stillwater I'm getting tired of it. But I've also been in Oklahoma my whole life, and although I love the state I'm ready to move on. However, I think that the town is fine and is improving every year. If you ever need to escape to a city Tulsa and OKC are both pretty close by (~1.5 hours to each I believe).
The weather in Oklahoma is crazy. It'll be 70 degrees one day and you'll have freezing rain the next (legitimately not an exaggeration - this happened when I was in high school and we were out of school for a week because of the ice storm). The summers can be really hot - last summer we had 63 days at 100+ degree weather, but that was a record-breaking summer. I don't think the humidity is ever too bad, but then again I like humid weather. While tornadoes do happen, it's rare that it'll happen to you. Most Oklahomans go outside and watch for them instead of hiding. Just know basic safety precautions and you'll be fine. In my whole life I've only had to take cover a handful of times, and I've only ever seen one in real-life once (and I'm the rare exception).
Overall, it's a cheap, nice school and town. I'm only undegrad, not a vet student, so I can't really speak for the vet school other than to say I really liked it when I was a pre-vet intern and that the students seemed happy!