I'm assuming you're asking about Apple's Iphone 3G. It's a great phone, but a phone is as only as strong as the network it's on. If you reside in a city like NYC, Washington DC, Boston or LA, you'll notice congestion on the network, where as if you were in a less densely populated area I've heard it much better. I also heard that they just doubled capacity in NYC for 3g coverage, but I can't confirm this.
On a related note there seem to be some complaints about operational sluggishness with the 3G when upgraded to iOS4. Apple is reportedly "looking into it" and in the meantime there are homebrew fixes being suggested by the masses. See the comments section in the link below for these suggestions.