I want to first say THANK YOU!! for such an awesome plugin!
Form customization can be a real nightmare on a site and this is by far the best plugin on any of the CMSes I've found for it.
My suggestion would be to add a "local inbox" for submitted forms so if email delivery fails or the user doesn't want to receive them by email, they can still use it. It would also provide a great way to keep them organized.
That's the one feature CF7 is missing that would be tremendously useful.
Thanks for all you do!