Not Recommended!
My wife's 3G runs like a hot, battery-hungry slug ever since switching to iOS4. She has to keep the 3G antenna off and only use Edge so that her battery can survive more than 3-4 hours.
My project for today is, in fact, to try to find a way to hack her phone back to iOS3.x -- a move for which there is zero support from Apple. If you ask the "Genius" Bar about it, they stonewall and suggest buying a new phone. Not really super-cool.
Not everyone seems to have a terrible experience on iOS4 with the 3G, but keep in mind that un-ringing this bell is a real PITA. Choose wisely!
Hope this helps you make an informed decision.