After struggling with even getting the User Profile Synchronisation service set up within SharePoint 2010 my syncs were failing every time. The ULS and event logs both let me down by not providing anything but generic messages.
Finally through much Google searching I found not the solution but a way to diagnose issues I was not aware of. The MIIS client, Synchronisation Service Manager. This tool can be found already installed (!) in %programfiles%\Microsoft Office Servers\14.0\Synchronisation Service\UIShell\miisclient.exe
Running this brings up a window with a log of all User Profile synchronisation attempts and more importantly their associated error messages as shown below:
This enabled me to quickly decide where the issue was and then resolve it, hope it saves someone else a few hours of searching for User Profile Synchronisation error messages!