You're NOT locked in yet. You've got 14 days to be satisfied or change your mind (but don't cut it close).
If you're having problems on WiFi, then obviously your phone is messed up. Have Apple check it out.
Coverage and tower placement are not equal between all carriers. One of the first things to find out (better before) is how well the coverage is for a new carrier in the places where you'll use your phone the most.
It's a radio. There are many variables which make each user's experience unique.
I've never had issues with other carriers in the past, nor have friends or family. That said, I've also used the phone in Minneapolis (definitely -not- the boonies) and had the same issues. I've had 3G on the screen most of the time, and have also been connected to WiFi. Bars average three unless I'm in the basement of our office building where signal's obviously bad... but I'll have 3 bars and a minute later the call drops.
I switched carriers to save almost $40/month and really regret it. Verizon's plans are ridiculous but now I'm spending $80 to have nothing.
I am somewhat close to you, (in Wisconsin) 54914 and have 3G almost all the time, yet everything is extremely slow. My iphone 4 with Verizon before worked WAYYYYY faster
You live a bit in the boonies. From the link above, you've got two towers in your "region". Maybe get near one or both and see if that affects your downloading and signal?
Poor battery life, inability to download anything, or poor access Siri on a regular basis tends to point toward lack of or weak 3G signal. How many bars do you have? Are you showing 3G service (see status bar). Your tower could also be down.
If you drop by an Apple Store, they can check your phone out for you. You've also got two weeks to decide whether to keep it.
I've had my 4s for 4 days - the battery life is incredibly short (~6 hrs with minimal-normal use), many Apps (iTunes in particular) are constantly "loading" and then fail, and Siri's response is 99% of the time "I'm really sorry about this, but I can't take requests at this time" (if that). Is this bad Sprint service or a bad iPhone? Either way: super frustrating and not worth the money spent on the phone OR monthly payments.