I know a lot of people have been using Emerald for a long time, but I just recently gave it a shot. So far I have to say I am enjoying this viewer a lot. The only other viewer I have tried excluding the basic SL viewer was Kirstens. Kirstens does have some nice features, but to me it just isn't good enough to be an everyday feature. Plus the best feature in Kirstens is the ability to render shadows, but that really requires a very high end computer to take advantage of. When I try I get one frame per second if I am lucky.

So I decided to give Emerald a try and it really does have a lot of great features I wouldn't want to go without. It adds so much and performs better at rezing objects and textures than the regular viewer. My personal favorite things it adds are double click teleports, a tab in inventory that shows what is worn, and the ability to turn off the particles for when editing prims. Another good feature it has is that I shows what viewer others are using. This can be good in protecting against those that use viewers that can copybot, and Emerald actually can warn you when one comes near. Lastly just for kicks I will say that Emerald adds breast physics for female avatars.

The website for Emerald has a more detailed list of the added features for Emerald here. So if you want a good performing viewer Emerald might be a good choice. I have heard of a few other viewers that are supposed to be good as well, and plan to give them a try. So far Emerald does seem to be the most popular viewer out. I will update if I find another viewer I enjoy more.