Europe is an entirely GSM continent, and your EVO's voice/text/data features will not work there, since it cannot connect to their network. Your WiFi will work just fine, and without a charge from sprint. You may want to turn off your mobile network, just to save on battery life since your phone might constantly search for a signal that it won't find. But you won't have to worry about coming back to the US with any roaming charges since the Evo will not connect to any towers
Check out a fun infographic that teaches you how to increase storage, enhance security, improve battery life and clean your device.