For us, performance on Dolphin was about as good as on the default PlayBook browser. It's not particularly fast, but in some cases we got slightly better page load times with Dolphin. We also didn't notice any stability issues, although we got the occasional error message when using Webzine. The interface, however, felt sluggish, especially when swiping to bring up the bookmark or add-on sidebars. Likewise, a few UI elements work particularly badly on the PlayBook OS — most of the Dolphin option menus, for example, are built to work with the easily-accessible Android "back" button, meaning there's no way to exit them without bringing up the PlayBook app toolbar. The gestures and other Dolphin features definitely help streamline the browsing experience, but we wish it had been better optimized.

Update: As commenter JCarty points out, you can also use a left-to-right bezel swipe to exit the menus, although we encountered a few responsivity problems with it.