I'm in your exact same shoes, i have a n900, i don't have a data plan

yeah everything works just fine, you can make calls just as you do normally, i connect to the internet only via wifi, and my wifi does get "3g" coverage, but i set it to 2.5g because it saves more battery, but i could use 3g if i want, the sound quality is even better with 3g in phone calls haha (but even at 2.5 it's better than most phones)