Well, it really happens two times a year, a new Ubuntu release.
The big change that a lot of folks will notice in 10.4 Lucid Lynx is a new look and feel, which you can preview here.
I like it, it looks like Ubuntu Studio. But it is always tough to gauge the success of visual GUI changes until you work with them, so the jury's still out on this for me.
Perhaps more important are the other new features of this release.
Well, that is where we sort of have a little problem.
This is LTS - long term support. So we aren't getting a bunch of brand new gee-whiz features (IMO of course).
Here is the official new feature list. Nothing to write home about in my opinion. I've been thinking about rebuilding my computer anyway, so I will be doing a fresh install of this. But if I were cruising along happily with my current distro? Well, since it is me, I would be upgrading. But for the rank and file user? You could probably stick with what you've got.