Whats memcached all about?

[Malcolm Frazier]

Hi,

I recently just heard about memcached and have a few questions;

1. How much memory will memcached "really" use?
2. What exactly does memcached cache? (What types of objects)
3. Does memcached cache objects per user?
4. If I have a high volume of users hitting my sites with PHP code is 
memcached going to eat up all my memory?
5. Once I implement memcached will I see a a big jump in performance?

Sorry I have so many questions, I am also very interested in hearing 
failed and success cases.
Thank you to anyone who can help.