I don't like the Form Editor as well, not so much because of the limited space, but because many features are missing : like the Signal/Slot editor, the View Code... menu item, etc.
If you go to Tools > Options... in Qt Creator, you can set the External Editor to be Qt Designer (by specifying a path similar to this : "C:\Programmation\2009.01\qt\bin\designer.exe %f"). Then, open the .ui file in Qt Creator. Then, press Alt+V+I (which is the shortcut for Edit > Advanced > Open in External Editor).
Qt Creator will automatically reload the Qt Designer modified file.