I have just started using Ubuntu after using Fedora Core 1,2 and 3. Fedora Core is a good linux and is extremely easy to install, but I was tired of downloading 4 CDs and getting 3 text editors, 3 cd-burning programs and 3 desktop environments when I only needed one.
I really like the look and feel of ubuntu as it sticks with Gnome and uses all of gnome strengths. (If you are a KDE fanboy use MEPIS linux) Fedora Core feels like it is patched together with duct tape as it uses KDE, Gnome and custom Redhat built utilities. For example, the Gnome System Tools used by ubuntu look much more ingrained into the Gnome experience because they are GTK 2.4 based, while the System-Config tools used by fedora are old and ugly (pyGTK 1.2 based I think). I also find ubuntu loads faster and uses less memory, but that could be my imagination.
In comparing APT vs. RPM, both are good package systems; however Synaptic for APT is slightly better than what Fedora offers in their "Add/Remove packages" and yum.
The only complaints I have about ubuntu are: it didn't detect some of my hardware so I had to find and add (modprobe) the drivers into the running kernel myself and it didn't detect my windows partitions so I had to add those to grub myself.