I have been beta testing Natty Narwhal for the past month, and so far I have been pleased with it. It did upgrade some key packages, but due to the graphics limitations on that PC that I have been testing it on, I have been unable to see what the new Unity interface looks like.
The replacement music player for Natty Narwhal is Banshee, and I like it better than RhythmBox, which was not as powerful. Ubuntu has also upgraded the python language that all of the programs built into Ubuntu have to 2.7 (it used to be 2.6 until recently).
Ubuntu still doesn't support programs written specifically for MS Windows, but WINE has been changing that for the past 10 years (at a slow rate however), and WINE is now into its 1.3 development stage.
I personally look forward to Natty Narwhal's release since it will mean that Ubuntu has consolidated the Netbook and Desktop editions, thus allowing me to use only one installation disk for two different computer types.
When Ubuntu 11.04 comes out, I would recommend that you upgrade to it if you use one of the earlier distributions of Ubuntu-stick with MS Windows for now if you have any software that will not run under Linux or WINE.