Yes, they are. I've used one personally when I was thinking of killing myself.
I rang a crisis hotline, explained my situation, and spent a good 3 hours on the phone with someone who calmed me down. I'm at the point now where I can reflect on it and realise it was just a massive bottle-up of emotions.
What I loved more than anything else was the aftercare. I had two ambulances sent over in the following fortnight, and whilst therapy was a given, I was also asked if I wanted to speak to other folks. The idea being, I can cry to someone who's been through it, but I can also speak to someone as I progress through therapy. It's amazing how much helping someone else helps your mental state.
I ended up with about 20 hours of therapy, a handful of ambo visits, and two friends I follow up with.
So yes - I'd say do it. Doesn't cost you a penny, you might make friends with people in similar situations, and for those who love you, there's an ambulance popping by every few weeks. I understand that those that are at higher risk get more frequent visits, but as always, it's free - and somewhere you can unload your thoughts in a safe manner.
I do think that if I hadn't sought help, and the help the NHS gave, I probabaly wouldn't be around much longer.