I recently install Visual Studio 2010 Service Pack 1. It surprised me when opening the Help a familiar-looking Help Viewer popup (although from the looks of it, it seems now rewritten in WPF following the visual studio itself).
I was a bit uncomfortable using the previous browser-based help initially, although overtime I am starting to get used to it and appreciate some of the benefits e.g: easier saving, simpler navigation. However, now that the desktop-based one came back, I am happy again :).
The main problem with me with browser-based one is that it got closed often and getting mixed up with other stuff opened in browser. It makes switching to it slower than desktop one since my mind seems to need additional work to get to it i.e: find the browser and select the one that open the help, compare to just open the application directly. The reason is quite similar to why I still prefer using thunderbird for opening mail no matter how improved gmail web-based email client currently i.e: desktop-based still feels more native and direct.