It wasn't me who tested, but from the horse's mouth, it is so-so. Unstable in the current versions of OS 4.2. The interface is decent and integrates with the Phone application (of course). Hopefully a lot has improved by next month, if that is, in fact, a firm release date for the OS update (at the rate updates have been issued, I'd personally hope this was pushed back to Dec/Jan).
Here are the screenshots that were snapped pre-service book (by moi). I'll see if BG will take some of the actual application and put them up.
Just an FYI, if you currently have an unlocked 8100/Pearl on Cingular, you can go ahead and get this service. It has been confirmed by BG as working on these as well. The release of the PTT software was unchanged from the initial T-Mobile OS to the Rogers OS, but an upcoming release does modify the file and fix some bugs/issues.