From what I can tell, it's the same as the bulk of recent releases. There are enough new/fixed/improved items that it is definitely beneficial to move to the latest version. However, it doesn't seem quite as pressing as before.
Part of that is because there are no real significant changes. There are some things that are very useful, like generating dynamic alignments and feature lines from corridors and labeling items in XREFs. And of course, we finally have our leading zeros in bearings! But there don't really seem to be any earth-shaking additions, or major changes to the C3D user interface, or major problems fixed.
And of course, part of it is because 2008 was far more stable than the previous versions. The program is definitely getting more-usable. I, too, have not really had it crash, but I have not been able to use it for much.
I notice some odd hangups, like hitting "Customize Workspace" in the CUI seems to take a long time before I get a response, and there are some weird pauses in menus and palettes displaying. And whenever I change the Working Folder, that dialog hides itself behind the main drawing window. Not sure how much of that might be due to the fact that I'm running it on Vista on a MacBook Pro.
Are you using the Menu Browser? I kind of liked the idea at first, but the vertical alignment really got to me after a while. It's too annoying going down the screen and selecting an item, then needing to go back up to the top of the screen to select the submenu item. I finally just turned the old-style menus back on. With those, the submenus appear right below the main menus, and it's much more convenient, mouse-wise.