Ubuntu's been greatly improved for this release, good thing to as it's the LTS. I did a clean install, backing up my home files and installing over my old 11.10 partition. This time I'm using 2 separate partitions (one for / and one for /home) so I'll be able to upgrade more easily next time.
12.04 has plenty of visual improvements, from the login screen to window decorations, though not all are in my favour. One feature I haven't yet got used to is how many windows are faded slightly when they lose focus. It sort of seems that you're only meant to look at one window at a time.
Speed improvements are the best part of this release I believe. Being an LTS that's important as more serious users may be sticking for 2 years at least.
Stability also appears to be good, though it's too early to say for certain. 11.10 never crashed for me, unlike 11.04 (but I won't say any more about that)
I'm still staying with Unity, and as a vaguely sideways implementation of the Windows 7 taskbar, I like it. I do use the smallest pixel setting because at 48px it's too big to save space. Then again, it never gets full up because I only dock Chrome & Nautilus in it, and use the Dash search for everything else.
One thing that has surprised be about this release is the HUD. Pressing Alt will bring up a text input that lets you search the menu bar. Now, just like the Dash speeds up open applications, HUD speeds up using the menu bar. However, it seems I never used the menu bar in the first place as I have not yet had the chance to use the HUD, instead the top tool bar of all my programs contains the buttons I use.
All in all, 12.04 is a great release, possible better than 10.10, but what might come in next releases?
Here are 2 of my predictions for what should come soon:
Smooth/anti-aliased window corners:
If you position the top left/right corner of a window in a while background, you'll see the jagged edges of each pixel. I'm hoping that this will be fixed in 12.10
Wayland replacing X:
It's been worked on for some time now and the old X window system isn't optimal for today's computers. Wayland should bring a number of improvements, not least helping the developers with a cleaner code base, so who's to say what will come of that? I'm thinking that Wayland will come in 13.04