The only experience I've had of mentoring so far is being mentored for charterhsip. My mentor was someone I knew of beforehand, but I didn't known very well. It's been useful to have someone with experience to bounce ideas off, and good to talk over little niggling problems. The only down side of the experience is that, being manically busy in an academic library, it can be really hard to get an appointment to see her - it once took me four months! So, my mentoring log is quite a short one!
I think having a mentor of some kind is a great thing anyway, and need not be related to something like chartering. Someone who has experience of the working world and can give you a different perspective sometimes. This doesn't have to be someone in your industry, just someone who understands the problems you might face, and these are often more universal than you might think; problems with colleagues, projects, office politics are all problems everyone faces at some point, regardless of where they work.
My sort of unofficial personal mentor is my other half - he has loads of experience working in offices of all kinds, and being freelance, of organisations and office cultures of all kinds. Any problem I face, he has faced it already: sometimes we face the same problem at the same time and we act as a sounding board.
The good thing about having someone to discuss all these things with makes you realise that you are not alone in the issues you face at work, on then other hand, you might say that these things never go away...