Probably I should try it before deciding (and I will), but I just don't see liking the global menu. First, I have dual 24" LCD monitors on my system (at work), so the one thing I'm NOT lacking is screen real estate. I have my desktop configured into 8 workspaces and I know generally what is in each one of them. On my main workspaces I have an Emacs window and some number, always at least 6 and usually many more, xterms. On others I have various GUI tools (none of which have lots of "child windows", so that doesn't bother me), and more xterms. I don't like Gnome terminal as it's too slow for me and takes too much memory: I like to have 2000 lines or more of screen history in my terminals and I often have 30 or more terminals open, to lots of different systems, at any given time. I don't like tabbed terminals because I often want to cut-paste between them or just look at the content of one while I work in another, or even just keep an eye on a build from one while working on another.
With all those windows I absolutely HATE click-to-focus. I always only and ever use focus-follows-mouse. Until a year or two ago I always enabled auto-raise as well but now I don't do that mainly so I can type on the "lower" window while reading the "upper" window.
I really can't see how global menus will do anything for me. Plus, how does a global menu work with focus-follows-mouse?!?! As you move your mouse up to hit the global menu you'll be changing the window that has focus... and changing the menus and which windows they control!
I have grave concerns. But, I'm confident that it won't be too tough to switch back to a more standard desktop experience, if I want to. And if it is, well, I used Debian for years before I switched to Ubuntu, and I can always do so again.