It's definitely a mixed bag. There are a number of definite improvements with Filemaker 12, but you also have take on slower execution/layout refreshes, new bugs and get used to the signficant changes made throughout the developer interface.
I recommend downloading the 30 day free trial, converting a copy of your database and checking to see what you get.
Advantages that come to mind:
The External Storage option for container fields makes for much easier management of inserted files over a networked solution.
The interactive optimization you can specify for a container field enables you to flip through the pages of an inserted PDF file directly in a container field--you don't have to open it in a web viewer or a PDF Reader.
Auto-Sort--where records suddenly jump to a different spot in the found set when you edit a value in a field that is part of the sort order is no longer an issue--you now have an option to turn it on or off when you sort the records.
You have some very nice "hover" format options that can change a laout object's appearance when you hover the mouse over it.
ExecuteSQL is a real game changer if you make the effort to learn SQL and how to use it in this function. A relationship graph for a FileMaker 11 solution might have 100's more table occurrence boxes than are needed for the same FileMaker 12 solution if the developer makes aggressive use of this function instead of adding extra occurrences to support "utility" relationships.
New options in the New WIndow script step allow you to make the window a "modal" dialog without having to do special scripted tricks to make it modal.
And the new tab control specific script triggers make for much simpler scripting in response to the user clicking different tabs in a tab control.