i used to work at 20 N. Clark (Clark and Madison) and speeds on my 3GS were similar to what you are describing at times - either slow or no data but then some days it would be great.
It was a crapshoot really, however, I would say that more often than not I would get very usable service so I think that it is worth looking into getting a replacement from Apple.
This aint good.
I'm heading back on the 10th for family stuff and ugh...if my signal craps out, all my friends with other carriers will just laugh................again.
Anybody else in the loop seem to have a battery that's depleting relatively quickly? I have 5 bars on 3G but data speeds are almost zilch. Trying to figure out what the deal is. I always plugged my 3G iphone in after 3 pm, but trying not to do that with my new phone.