Yes...I currently play with 2 GTX660 cards in SLI. I definitely notice a difference when SLI is disabled (usually happens when I do driver updates and forget to enable it again).
Well, now that we have full-screen back, when can we expect to get an SLI profile? Yeah I know, that is a question more likely to be answered on the Nvidia forums, so I left a post there as well.
Nvidia released their 340.43 beta drivers today. It added SLI support for ESO among other tweaks. I wonder if it may also help with problems that some of you are having.