I am currently upgrading to Ubuntu 8.04 and so far so good. For the past releases I have upgraded from the standard server but that was very slow (8 hour download).

Fix: Use another server.

My current download is almost complete and my download speed is currently at 164kb/s (my max, thanks AT). To change the download server, simply open System > Administration > Software Sources and select download from other. Pick a random server in your country and hope for the best (the pick the best server button does not find the least bogged down server, it finds the closest server). The Ubuntu teem should make this happen automatically. The System Updater should find the mirror with the greatest download speed and use that server for its upgrade.