I am using the Linux version and although it is in Beta I feel that the viewer has certain design flaws which won't be fixed for a few years.
The program also feels kinda bloated with all kinds of crap in there like a web browser. The idea is that you never have to leave the second life viewer and want to stay in the world 24 hours. I wished the viewer was just for viewing second life and they focused on that rather then including so many features that everything is broken.
The game has a lot of UIs which can put off new users from trying out the viewer.
Its very difficult to import your own objects from blender, 3D studio and other applications into second life. With each attempt I tried I ended up with an object with the verts all messed up. I wished they worked on an importer so you could upload reasonably sized objects without having to change them into a texture. That whole part of the system feels like a hack job to get a new feature.
Even with all these problems second life is an awesome idea and I love spending time there but I believe that problems with the viewer really slow down adoption rates.
If they could simplify the User Interface a little more then they have and work out all the problems it would be a great place to hang out.