I have upgraded to Ubuntu 9.10 Karmic Koala and have been extremely happy, it is the most stable and workable version of Ubuntu Linux to date. However, there is some sort of bug with the network connection. There is a lot of discussion around this at launchpad, but there is a workaround that really works…so until it gets fixed change your DNS setup by following the directions at OpenDNS.
https://store.opendns.com/setup/device/ubuntu/
I followed the directions (these seem to be for Jaunty but you should be able to figure it out) and the connection reset and the problem just went away.
BTW this is not an IPv6 problem like the one we reported with Hardy Heron 8.04