I have been using it off and on for about 6 months now. (last two months have been using it as my main desktop development at work)
I like the desktop cube because it gives me a sense of direction when I am moving between desktops. 4 desktops on a flat plane and I get lost when I am switching between several apps and trying to remember which one they are on. With the cube it is just a bit easier to associate which desktop is to which and where an app is that I left around. edit: When having the cube set to transparent when rotating, you really then have a better awareness to every desktop. When you move from one to another, you have a visual representation and a location of where apps are. This for me really helps as I move around desktops quite often.
I really do like the "All Windows" thumbnail preview though. I have it set when I move the mouse to the top right of the desktop it thumbnails all active windows. This is the second best feature and when you have several or dozens of active windows, this feature is many times better then ALT-TAB'ing around trying to find a particular window.
I also really like the real time ability for full window transparency. ALT+Scroll Wheel will change the transparency of the entire application. This is great for so many reasons. Transcribing something from the background window without having to ALT+TAB back and forth, keeping an eye on some resource while working on a project, watching a movie while doing some php coding. (kind of unproductive, but it works :) ), multiple rdesktops...
There are a few other items that I like that enhances my productivity, although not to a great extent. The rest is just eye candy, and I leave them on in a minimal state just to help convert the Microsoft users over. They are speechless because they think only Mac and Vista can look this good :KS
And I have never had a lock up. It just seems that every time I try to upgrade to a newer version I get the white-screen-of-death :mad:
edit: I also wanted to add that yes, XGL/Beryl will use more resources. But why would I buy a dual core computer with 4Gigs of RAM and only try to convince myself that it is okay to only use 10% of it's power? You don't buy a sports car with 310HP and make it a practice to use only 31HP when you drive it do you?
Cheers :)