Functions or Rules and Views?

I'm embarking on a new project and have been reading up on the new v7.3
functions. I have a few general questions about how to use these and
whether or not I should use functions exclusively, or also use rules and
views.

For instance, it seems easy to wirte functions that write values to two
tables on an insert, obviating the need for rules. It also appears that
a function can replace a view.

Are there any penalties in terms of the speed of execution of a function
over a view?

Some general comments would be greatly appreciated.

Thanks,
Rory

--
Rory Campbell-Lange
<rory@campbell-lange.net>
<www.campbell-lange.net>