Having used it for a few days I'm in two minds about Ubuntu 11.04.
This is reportedly the last version that will support the "classic" GNOME 2 environment. Having used GNOME 2 since I switched back to GNU/Linux from Mac OS X half a decade ago, I'm sorry to see a good friend go. And GNOME 2 really is a good friend, probably the best user interface anyone has put together for Unix (well, aside from Mac OS X, but I'm not sure if I really feel that counts.)
GNOME2 is intuitive, quick (on my hardware), slick looking, friendly, and the builders of it took the time to understand concepts like Fitts law and muscle memory.
Natty gives you the choice between GNOME 2 (called "Ubuntu Classic") and an environment originally intended for netbooks, but substantially improved, called Unity. And Unity is...
...well, it's not ready yet, really, is it? Now, don't get me wrong, it's got a huge number of excellent ideas. Some of these are clearly borrowed from others - the dock, for example, is clearly inspired by the equivalent feature of Mac OS X. The top-of-the-screen menu appears to owe much to the Amiga environment, funnily enough - displaying menu only when it's needed, but title information otherwise. And we still have many classic GNOME elements, such as the Nautilus desktop/file management environment, and virtual desktops.
On the other hand, good ideas or not, sometimes the whole can be spoiled by a few glaring issues, and that was my problem with Unity. In fact, the major issue is the way to launch applications that are not in the dock. I think the assumption made by Unity's developers is that if a user uses an application a lot, they'll put it in the dock, but in reality the dock just isn't large enough, and you still have to start somewhere. Launching an app means going through a system that's laid out like the search feature of a really bad retail website. Finding the apps you want to run requires an enormous amount of clicking around.
Still, the system has enormous potential. If it's true that the next release of Ubuntu doesn't have GNOME 2, then it'll be interesting to see what the result is. A fixed Unity would make the next Ubuntu awesome. The danger is that it won't be.