My statistics for the last 30 days on my Lumia 1020 Windows 8 Smartphone:
Mobile: 77 MB
WLAN: 136 MB
I don't phone often. Maybe one or two calls to my parents per week. I rarely text.
I use calendar function occassionally, as well as OneNote, and I synchronize my phone's images with OneDrive (but I don't make that many photos - but it's a Lumia 1020, so the photos are bigger then for most smart phones.)
The major app that I use regularly currently is Chess Time, to play chess with friends.
At home and at work I can use WLAN (though at work, I am usually not logged in there).
The Map app (Nokia HERE) I use allows me to download the maps to the device, so I can do that at home in my WLAN and only need GPS for nagivation.)
I have a contract that gives me 4G speed until I exceed a 500 MB limit/month, then it gets downgraded to I don't know what. No extra cost occur, but I can order extra packages for another 500 MB.
I have no free calls and no free texts - it's 0.09 € per minute of phone call or 0.09 € per text.
The base price is abotu 10 € per month.
I did buy the phone seperately without contract, which is expensive - but remember, contracts where you get the phone cheap or for free will generally contain the cost in the price over the minimum contract duration. (And will likely auto-renew, so you are not flexible either.)
The smartphone is definitely a luxury, I don't strictly need it, but it's convenient enough that I think it's worth the expenditure.
I can't offer much more insights, since the providers in the USA and Germany are not the same (other perhaps than T-Mobile). Coverage with my provider (basically a reseller of T-Mobile/Deutsche Telekom) is good (and there are no providers that are really better, just one that is about just as good, and worse). This may be a bit more difficult in the US, as far as I know, coverage even of the big names isn't always great. (I suppose it's excusable if you consider how big the US is and how the population is distributed.)
