I wasn't a huge fan of it at first, but it does have it's merits. The interface is nice, and has some great concepts such as on-demand multiple-desktops, centralised areas for applications, etc. I do look forward to it's eventual stable release. To be honest I haven't had any major issues with it so far, no stability issues.
The problem is that Compiz may load anyway in Gnome3 if it happens to be enabled in Gnome2 as the Window Manager. Compiz doesn't work with Mutter (The evolved form of Metacity, another compositor).
To make matters worse, there isn't a way to use Gnome3 with the NotShowIn= command under home/yourname/.config/autostarts/insertautostarthereforcompiz.
What needs to be done is to code something into distros that
A: Shuts off Compiz and enables Mutter when Gnome3/Gnome-shell is selected as the Desktop Environment (I guess mutter --replace might work)
or
B: Something that allows you to force specific Window Managers for Specific Desktop Environments, and not let others load.
By the way, this is not the version of Gnome3 that you'll currently get from repositories. The one pictured in the Gnome3 webpage is the relayout branch. The one you'll currently get from a repository is the preview version, which looks like this.
Gnome3 Preview
To get the relayout branch, you'll need to get it from git.
As for Unity, Compiz will live on as part of it, as Unity is being coded to use Compiz rather than Mutter, it wont die. Unity isn't my favorite cup of tea though, but whatever works I guess.
Oh, and Gnome2 isn't going anywhere for awhile, if you install the Alpha builds of Natty Narwhal, or upgrade from 10.10 to 11.04 Alpha 1, Gnome2 is still included, but it's listed in GDM as Classic Desktop. Even if Gnome2 ceases to be included in Ubuntu or whatever, remember there's always XFCE, which is more lightweight, supports alot of Gnome2's components (Including Compiz), and to add onto that it probably will not change anytime soon.
It'd be so awesome if the Compiz Developers created their own Desktop Environment based on Compiz.