Does it help to use NGEN?

NGen will only help startup time - it doesn't make the code execute any faster than it would after JITting. Indeed, I believe there are some optimizations which NGen doesn't do but the JIT does.

So, the main question is: do you have an issue with startup time? I don't know how much of an ASP.NET application's start-up time will be JITting vs other costs, btw... you should probably look at the Performance Manager graphs for the JIT to tell you how much time it's really costing you.

(In terms of availability, having multiple servers so you can do rolling restarts is going to give you much more benefit than a single server with an NGENed web app.)

NGen isn't the way to go for ASP.NET -- the creation of the .dlls in the bin folder isn't the final step -- they are compiled again with the web/maching.config settings applied into your C:\Windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files folder. Instead of NGen, to decrease initial load-time, use the Publish Website Tool or aspnet_compiler.exe

I'm not sure that NGEN's primary benefit is to start-up time alone - if the application's suffering from a high '% Time in JIT', this is listed as a potential remedy: http://msdn.microsoft.com/en-us/library/dd264972(VS.100).aspx.

The discussion is closely related to a different question on how JIT'd machine code is cached and re-used?