I recently upgraded several PCs to Ubuntu 12.04, either from 11.10 or from 10.04. Most upgrades went just fine apart from one, on the Dell Latitude D505 which for some reason didn't download half the updates and had to do a sudo apt-get upgrade afterwards to get them all installed. It is apparently not recommended to upgrade from 10.04 to 12.04 until 12.04.1, though I had no problems doing so on one PC I tried. On my main desktop I did have to change my Gtk theme to Adwaita since the theme I was using (Ambiance Blue) has not been updated to work with 12.04 yet, and is simple to switch using Ubuntu Tweak.
I also tried the standard Gnome desktop, it is slightly better than Unity in some ways, apart from it needs decent hardware acceleration and the virtual desktops don't work properly on dual monitors (switching only works on main monitor). It also seems it needs more tweaking than Classic to get it how I would want it and in Gnome I miss my wobbly windows! The only thing I like about Gnome Shell and Unity is both have a good search function, it's easy to search for apps and files, other than that they both miss the mark for me. The search function in nautilus is fine for searching for files for me at the moment.
Overall, 12.04 brings quite a few other improvements over 11.10 such as better battery life, the new Privacy Manager feature in System Settings, and many others. In fresh installs, Tomboy has been removed (I've recently switched to using Evernote instead so I probably won't install it on fresh installs), and Rhythmbox has replaced Banshee as default music player. This does not bother me since I mainly use Gmusicbrowser which is far quicker and lighter than either of those, it scans my large mp3 collection in no time at all, unlike Banshee and Rhythmbox which seem to take hours! Ubuntu 12.04 is a solid LTS release, and well worth upgrading to, though I (and Canonical) would recommend waiting until 12.01.1 to upgrade PCs running the previous LTS, 10.04, or consider fresh install if you cannot wait!