I got mine years ago at the old LHBS for 5G batches in a flat-bottomed enamel pot (the glass Pot-Watcher in the wiki article), but I haven't seen one since. It seems to really help, especially with situations where you would expect a sudden boil and boil-over, like high gravity boils. I used to do my mead batches with a 15 min boil to skim albumin, and that high gravity sugar seemed to get out of control fast, unless the Pot-Watcher was rattling away in the bottom. And it works great for regular cooking, too.
We'll see how it works with 10G boils in a keggle. Different shapes and bigger volumes may negate it, but I bet the round bottom of the keggle may keep it centered where it needs to be, and the keggle's relatively narrow column/footprint may not be an issue.
Keep in mind, a centered dip-tube would get in the way in some keggles, but it's definitely worth the couple bucks.