If you are developing JavaScript or ecmascript based application for SharePoint 2010 which will be using Dialog Framework SharePoint 2010 you have to write JS. So being a server side code from long time I do not think of living without Intellisense. Look below on how you can enable IS to your App page or Visual web part.
Instead of me copy pasting from the MSND Article. Mind a click below and watch it for yourself. i was looking for Visual Web part for my case.
How to: Enable ECMA Client Object Model IntelliSense in Visual Studio 2010
