I think you may get better dispassionate advice on here. Like all these things, I suspect it very much depends on who you get. Have I been on the dark side, yes. Have I called them, no - but I know a few who have.
No, I've not called them but I worked for a university student version and did some training for them and had some training myself from the Samaritans for it.
I've never rung them although I've had some tough times in the past. A few years ago my mother staffed their phones in Amersham one night a week for a few months. She said that all she was allowed to do was just listen - she wasn't to engage in conversation or give anyone advice. All she could basically do was say "Yes", "No" , "Oh I see" etc. Just be a voice for someone to speak to rather than engage in conversation.
I can understand the not giving advice bit but I know when I was very low it was someone giving advice that I actually needed! If I was speaking to someone who refused to answer questions or give advice then I would have got frustrated and put the phone down even more desperate and worse than I was!
I'm sure the Samaritans help many people but there are also many people who can't get the help they need from a disembodied voice on the phone that won't give them any advice. The problem is that those are the very people who mostly call the Samaritans.
When I was with them, we did have some people come into the centre who we saw and needed help, but of course it was mainly phone calls that people made. Didn't have much feedback as to it helping people, but some seemed grateful to be able to unload problems which must have helped having someone impartial who would listen.