My only experience of ‘official’ mentoring was when I did my chartership. I never actually met my mentor, as she lived in a different county, and we usually communicated by email, and once on the phone. I think the distance between us made it difficult to form a good relationship, but I must also admit that I wasn’t great at keeping in touch with her.
As for ‘unofficial’ mentors, there are people I’ve worked (and still work with) who I look up to and try to emulate, but I wouldn’t class these people as mentors as such. I can see how mentoring can work well, but I’m not sure I would make good use of an ‘official’ mentor at this point in my career.