Well everything may be good and may be evil based on how we use it. From my experience consious and judicious use of UpdatePanel is the key to saving its benefits and avoiding potential problems.
Below I suggest a number of rules that help achieving better results when using UpdatePanel.
- Avoid automatic refreshing of UpdatePanel; always stay in control of which UpdatePanel and when refreshes: set UpdateMode property to Conditional (the default value is Always).
- Minimize the content of the UpdatePanel: the <ContentTemplate> should only include controls that are neccessary to refresh. For instance if user input requires server-side validation include only an error message mark-up in the UpdatePanel and leave the rest of the form outside.
- Try to keep the partial postback trigger controls outside of their respective UpdatePanels unless its neccessary to change their markup.
- Try to stick to a simple rule: one trigger for one UpdatePanel. If you need to refresh multiple UpdatePanels during one request add a trigger control to only one of those UpdatePanels and refresh the others programmatically in an event handler on the server. The idea is to avoid uncontrollable refreshing of unnecessary UpdatePanels.
- Since ViewState is updated with every partial postback request turn the ViewState off on a page that contains the UpdatePanel wherever possible or store the ViewState on the server to avoid transferring it forth and back with every async request.
- Since Page runs through its lifecycle during every partial postback and executes methods like Page_Load or Page_PreRender make sure that logic that is unneccessary for refreshing UpdatePanel is not executed by wrapping it in if(!ScriptManager.IsInAsyncPostBack).
- If you use UpdatePanel event handlers like Init, Load, PreRender and Unload make sure that code inside these event handlers does not execute unless neccessary by checking Page.IsPostBack and ScriptManager.IsInAsyncPostBack properties.
- If you programmatically update Page's Header (Title, etc.) or other Page's content that is outside UpdatePanel make sure that this code never gets executed during partial postbacks. First of all its not neccessary since page does not refresh but also it may be dangerous because the content may not be handled properly by a browser.