I've been using the Emerald viewer for quite a while now - the implementation of breast-physics was just too good of a feature to pass up. However, there are a few things that have now tempted me away from the jigglyness.
1. I started to experience more and more Transaction errors with Emerald. I'll get a message box that will say something to the effect of "your transaction could not be processed." Shame on me for not capturing the exact text for you. But the end result is that I lose a linden here and there, while my balance temporarily goes to -1, requiring a relog.
2. The introduction of Linden Labs' SL 2.0 Beta viewer. There are several new features being introduced with this new viewer, some of which include:
a. Browser-like navigation for teleporting
b. A new sliding-menu motif for the main user interface
c. A separate tattoo-layer for avatars
d. An alpha-mask layer for avatars (no more invisiprims)
e. The ability to apply shared-media as a texture to a prim face. Not only video, but also browser sessions
I should re-iterate that the SL 2.0 is still in beta release, so expect the usual bugs, fixes, and changes. They've done a major overhaul on it, so you'll no doubt be fumbling around trying to find new ways of doing old things. Performance seems to be (for me) about the same for this viewer compared to Emerald.
If you're an early-adopter, or just curious, give it a look.