I disagree strongly with the accepted answer here (but I'm currently implementing one, so I'm biased). As for research/support, there's been a number of articles published on this topic, by Don Hopkins in particular. Check this video out, too. Here are some highlights from Don Hopkin's original study on Pie Menus:
Seek time (shorter = faster):
(source: donhopkins.com)
Error rate (shorter = fewer errors):
(source: donhopkins.com)
The study's conclusion tempers the results somewhat:
What does this mean? Should we program
pie menus into our bitmapped window
systems tomorrow and expect a 15-20%
increase in productivity since users
can select items slightly faster with
pie menus. Pie menus seem promising,
but more experiments are needed before
issuing a strong recommendation.
First, this experiment only addresses
fixed length menus, in particular,
menus consisting of 8 items - no more,
no less. Secondly, there remains the
problem of increased screen real
estate usage, In one trial a subject
complained because the pie menu
obscured his view of the target prompt
message. Finally, the questionnaire
showed that the subjects were almost
evenly divided between pie and linear
menus in subjective satisfaction. Many
found it difficult to "home in on" a
particular item because of the unusual
activation region characteristics of
the pie menu.
Advantages:
- Better for repeated use as they can be learned in muscle memory
- Faster to select a particular item
- Lower error rate (especially when implemented correctly, such that the hit target for an area is the entire region)
- More natural for sub-menus
Disadvantages:
- Implementation
- Harder to scan
- Large; can cover up content
- Max 12 items
- Bad for variable-length lists or items that may move in position
- Unfamiliar
- Seem not to appeal to "techy" users
Successful uses of radial menus:
- Many video games, notably The Sims series. This game is worth playing if you're thinking about them, just to see how much they improve the experience
- Maya, blender, and other 3D packages
- Mouse gestures in Opera and other browsers (invisible radial menu - but basically the same thing)
- While not radial, the windows start menu's nonlinear design. On Win7, from clicking start, I can very easily access my pinned icons, "All programs", the different "places" on the right, shut down, or the search box
- Ditto office 2007, in particular the set of formatting tools that appear above a selection. In fact if you right-click on a selection in Office 2010, you get:
alt text http://nv3wrg.blu.livefilestore.com/y1pFJ1efW2AgTLJtf-4WcxhlDbPRvVdYG5J4t5lYUtzzmuWIgbTXr028bF0mKeViLwYU6tJf_d7XVpreyJKAl0aAZpJFnBINMOn/offmenu.png?psid=1
A nonlinear menu! Not radial by any means, however many of the important options are distinguished by angle from the cursor as well as distance.