I already blogged about Gnome Shell, however it has changed a lot since then.
In short, I used Unity for about an hour using live CD and my impressions were mostly negative - I found Gnome 2 being better for me. In case of Gnome Shell, I used it for some time in the past and am using the final release for some time now. I also tried final version of Gnome Shell via live CD before upgrading to Fedora 15 and impressions were mostly positive.
Unity impressions:
- Positive things
- Looks like more of screen space is used for usefull part of window compared to Gnome Shell
- Drawbacks
- Too much stuff in dash
- Not completely clear, what the button in top left corner is supposed to do
- Application launcher is not comfortable: it appears to be based on search function, browsing categories via drop down is usability drawback
- Places: same issues as with application launcher
- Workspace switcher based on window movement, I like GNOME Shell version better - I use this for moving window to other workspace, not for changing it's coordinates
- Not sure I like the single menu at the top panel
- Alt+Tab is not very usefull with all windows from all workspaces being there
Gnome shell impressions:
- Positive things
- Overview mode is great
- Alt+Tab is awsome
- Workspace management is great
- Notifications at the bottom of screen are very comfortable
- Drawbacks
- All application view is bloated and takes long to load
- Pressing Alt to shut down is both unintuitive and unuseful
To summarise, I like Gnome Shell and certainly believe it's better than Gnome 2. Although I have used Unity for only about one hour, but that was enough for me.