We've got a page with a ton of jQuery (approximately 2000 lines) that we want to trim down b/c it is a maintenance nightmare, and it might be easier to maintain on the server. We've thought about using UpdatePanel for this. However, we don't like the fact that the UpdatePanel is sending the whole page back to the server.
I don't know if there is a way to optimize UpdatePanels, but my company has found its performance to be pretty poor. jQuery is much much faster at doing pretty much anything.
There can be a lot of lag between the time when an UpdatePanel triggers an update and when the UpdatePanel actually updates the page.
The only reason we use UpdatePanels is because of the ease of development. Almost nothing needs to be done to make them work.