I've experimented with enabling shadows in the Snowglobe client. A lot of the problems I used to see have been corrected, and the shadows are beautiful.
There's one big problem that I've noticed: snapshots with shadows enabled are still solid black. The preview window just shows the sky as black (and if you look at the saved image of the last place you were before logging out, the sky's black there, too), and the saved image is solid black. I had to use screen captures to save what I saw--which, after I spent so much time arguing that SL "photographs" aren't just screen captures, is kind of ironic.
The other thing that I've noticed is objects coming and going; plants and strands of hair vanishing and reappearing. I can only conclude that enabling shadows ups the number of vertices to be processed enough that even after increasing the arbitrary limit imposed for RC/Snowglobe, it still goes over, so I have to increase it yet again.