It's not that bad, it's like dial-up speed which for looking at text and stuff is fine.
I switch the 3G off when I'm at home because the reception is poor inside this little concrete bunker of an apartment. Full GPRS reception seems to be faster and uses less power than flakey 3G.
Not all the networks down here have Edge (five networks offer the iPhone in Australia and Apple sell them outright, unlocked through the Apple Store and Online Store) so when I turn off 3G, GPRS is what I get.
Doesn't bother me, all I'm doing is looking at the MR forums, Twitter and NetNewsWire which is all basically text so it comes through fast enough.