Is it worth using AMD Fusion while gaming?

Depends, to be honest. It shuts down all processes except for the essential ones along with any background tasks that were going to happen. I've used it to do a restart of my computer without having to go through the full loading. Once you have it tuned right it can give you some substantial improvements to your gaming. To use the utility to it's fullest you need the AMD OCing tool and ATI Overdrive and have them both setup.
When I play Far Cry, Fallout 3, Deadspace, etc. I usually turn it on because it lets me play at stable frame rates at a better resolution.
There was also a site that did some testing on the Fusion application and it showed 10% to 20% increase in computer performance depending on the game.
Found the article: http://www.extremetech.com/article [...] 988,00.asp
Note: You can use it on Visa 64 computers and averages look to be a 10% to 20% increase in performance.