Facebook applications have access to a service they call Proxied Mail. It's basically an email address unique to the user/app pair and forwards to the users registered email address as long as they continue to grant the app permission.
So I can use this and prompt for permission, or I can just ask the user for their real email address straight off.
Which would you consider a better user experience?
I'd personally prefer to give an email address once, but why not give users the option?
Default to asking for a real email adress, with a "Don't want to use your real email?" link as well that briefly explains the downside to not giving one.
Prompt for permission. If they agree, they have to click on 'Yes', which means one click, as opposed to typing an email address. I know both actions only take at most 10 seconds, but users don't want to spent any time entering profile information.
If it's UX (User Experience) you're worried about, I'd go with 'asking permission' for two reasons:
It's less work to give permission than re-type my email address all over again. It also keeps the sense of a centralized application rather than disperate applications attempting to work together.
It also gives the user a sense of security. At any time, if they want to stop receiving email from you application...all they have to do is take away permission. If they were to enter their email address, who knows what you might do with the data (whether or not you'd do anything is irrelevant, it's what the user experiences at the time of signup/configuration).
In terms of user experience I feel unnecessary (invasive) prompts are bad.
So, when making a decision on this UI try to give the user as much control as possible. Users in control makes for a happy user experience.
Have both options available on the screen / page. Let the user choose. i.e. Enter your email address here..... or, "check here to permit access to stored email..."