A few years back, after I installed Vista, I spent quite a bit of time trying to fix my GigE Ethernet performance with Windows Vista talking to Windows Server 2003 R2. My file copy performance hovered around 15 to 18 MB/sec, which was pretty dismal.
I've just built myself a new PC with a Core i7 CPU and Windows 7. I tried copying a file from the old Vista box (Core 2 Duo 2.4GHz on ASUS P5B Deluxe mobo) to the new Windows 7 box. The copy performance over the Ethernet went straight up to 50MB/sec! This was with a single WD Caviar Black drive on the Vista system, no RAID or striping.
When I tried copying from a Windows Server 2008 R2 system to the new Windows 7 system, I peaked at 112MB/sec for data that was cached, then backed off to 50 MB/sec for data that wasn't cached. Windows Server 2008 R2 was installed on the same hardware that used to be running Windows Server 2003 R2, so the performance increase was solely due to the OS upgrade.
I'm seeing similar performance gains over the internet. Talking to our corporate server from the Vista system, I maxed out at 1.5 MB/s. Under Windows 7 with i7, I'm able to max out the connection at 5 MB/s.
All of this leads me to believe that the GigE networking performance of Windows Server 2003 R2 was awful, given that I'm running Windows Server 2008 R2 on the exact same hardware with a 5x performance increase.
Net result: (no pun intended) I'm very happy with Windows 7.