It works well. I've been using it since it was released this week. The cool thing is that fonts activated are acccessible in both Carbon, Cocoa, and Classic apps, which is quite nice.
As pointed out, the application needs to be open for fonts to remain activated. I was kind of surprised Extensis chose this route instead of a lower level System Preference item. It doesn't make much difference though, the application sets itself to launch at startup, and it's just another Icon in the dock. You can even hide it at launch.
Another nice feature is that you can activate your font sets through the Dock icon, much like you can in OS 9 through the Suitcase CSM modeule.
All in all, I'd give it 4.5 out of 5 stars. It'll get 5 stars when Extensis makes it a background application with a Menu item activation icon.
I used to be a ATM Deluxe supporter, so I was definitely disappointed when Adobe decided not to support it under OS X. However, Suitcase 10 for X is as better than the last ATM Deluxe iteration (4.6?).
Definitely worth the $99...