First impressions (and I'll be brief as this has been written about ad nauseam) are that the slippery plastic is actually quite nice - slips into the pocket more easily, seems lighter (I'm not sure if it actually is) and is yet sturdy in the hand.
So, pick to the fore, SIM out of the old, SIM into the new, sync with iTunes. Boot and...
1 bar on the phone. Now, the old phone was giving me 5 as it sat next to my computer. Wave it around a bit. Still 1 bar.
Read some of the (few) posts that I had not previously browsed about the new iPhone... terrible 3G performance, particularly in the UK.
Turn 3G off.
Long-running test (5 hours and going) - max reception approached: 3 bars, somewhere near Eight Over Eight on the Kings Road on my cycle home.
Please tell me it's O2's fault as then I just have to wait for new or upgraded cell towers. If it is a hardware issue in the new iPhone then I am very, very disappointed.