I had the opposite experience, each update over the last 2 years made my 3G feel faster (in some cases measurably so).

Excluding 4.0 that is, most stuff got slower then.

TomTom was always painfully slow on the 3G though. TomTom is just great on the iPhone 4, faster than any dedicated TomTom unit.
_________________________
Remind me to change my signature to something more interesting someday