FP1 is nice but I wouldn't worry too much if you don't have it.

The main plus point is the S60 browser has been upgraded, it now lets you save passwords, run flash within pages, and allows you to rotate the screen so you can view pages in horizontal mode. The horizontal mode is the nicest thing, it makes viewing pages much easier than before, especially if you also zoom out and set fonts to small.

Other nice things:
-You can create folders and sub-folders (and sub-sub-folders etc), and place any icons in any folder at any level, so you can structure the UI more to your liking.
-The settings menus have all been placed in a single section, which saves a lot of bother.
-There's a blue ring which appears next to app icons which are still running (or next to their folder if you're at a higher level of the folder hierarchy). This is quite handy if you forget to exit applications, it reminds you to switch them off.
-There's a couple of 3D modes for the menu system. They look nice but they're not that useful, I switched back to the old grid view fairly quickly.

Incidentally, there's an FP2 announced which will have more radical changes, but it's not due to enter service until well into 2008.