Sprint's coverage map. Uncheck National Network, check Mobile Broadband, start zooming. Basically you'll see broadband coverage is quite good but not as pervasive as voice alone. I've heard that Sprint now has roaming data coverage but haven't bothered to check.
I've been a Sprint customer for years (8+ IIRC) and the coverage areas have only gotten better over time. My current plan includes PowerVision (EV-DO) with Phone-As-Modem. PAM ups the cost a bit but eliminates the need to do a device hack to use the device as a modem. Also, w/out PAM using your PC is a TOS violation and your account could get shut down if found out (it happens). PV is $15 a month; PV w/PAM is $39.99 a month on top of the phone plan, which still beats standalone data-card accounts (last I checked they were $60-70 a month). Work pays for my plan, which of course influenced my choice. Were I the one paying I might take a chance.
As DD said, speed is good, basically like DSL, but latency is higher. If EV-DO is not available, it automatically falls back to the old 1xRTT speed which is still 70-140Kbps; about twice modem speed. But in Chicagoland EV-DO is pretty pervasive and I average 600-900Kbps on speed tests.
As a Treo user I use the bandwidth for email & surfing on my device and also as a cellular modem for my laptop when I travel & WiFi is unavailable. Google has GMail and GoogleMaps apps for the Treo. GoogleMaps now integrates with Contacts to let you map a contact's location directly.
DD, Sprint hasn't blocked BT connections for Phone-As-Modem, but while I've done it before there's definitely a performance penalty compared to connecting the phone to the laptop via USB. I keep a retractable cable in my laptop bag.