Second Life Viewer is the first viewer that most people will use when joining Second Life. It is developed by Linden Labs who also develop Second Life. It provides all the functionality required by a new user of Second Life but seems to lack some of the features that veteran users may want to see and use. The basic menus and functions are easy to find but the more advanced ones seem to be harder to locate.
It seems to run just as well as the other viewers that I have used although I did not use Second Life Viewer for that long or go into populated areas but since it is developed by the creators of Second Life I would assume that it would run as well as any other viewer.
My impressions of Second Life Viewer are that it would be a good viewer to use when first joining Second Life once they start to get some experience and knowledge about Second Life another viewer with more functions and option may be a better option.