I find it really, really good. I've done several courses on mindfulness run by the mental health team, local adult education centre and there's some stuff about it in the degree I'm doing (health sciences), and the scientific research that's going into it is really interesting, with creating new neural pathways and such, which can really benefit those with all kinds of mental health problems. I now do a yoga and mindfulness class once a week, which helps keep me on track with practicing, as sometimes I can get a bit complacent when well and then I find it harder to access a mindful state if I'm upset.
I use it a lot at work. I'm allowed to take extra breaks if I need them because of my mental health condition, and often I'll pop outside and stick a quick Headspace meditation on (hooray for being a uni student and getting free Headspace with Spotify!) for 5 minutes or so, just to ground myself and prevent myself from getting in a bit of a state. I find that it's really improved my performance at work, in that I'm able to concentrate better and I am a hysterical wreck less often (it doesn't work all of the time, and I still have the occasional breakdown, but they're much less frequent.)
I struggled with some mindfulness because of the silence that is suggested sometimes, because when Im unwell (and sometimes when I'm well - they just pipe up for the sake of it), I hear people that I cannot see. If I feel myself starting to get distressed by them and I'm at home, I've got a guided meditation on my waterproof MP3 player, so I get in the shower and have a mindful shower with the guided stuff, so I've got something else to listen to, in the middle of my head (you know how when you listen to music through headphones and the music sounds like it's right in the middle of your brain - I find that really helpful when I'm being yelled at by invisible folk.) It doesn't always work, and I still end up getting in a state and calling crisis or my CC or duty sometimes (who are always lovely to me and very helpful - they have a crisis plan on RiO that they go through with me, and it does not mention telling me to make a cup of tea and have a bath!), but like with work, those instances have reduced significantly.
I don't think it's appropriate to suggest using it as a one off in the middle of an acute mental health crisis, but I do think it's a really positive thing that, if practiced enough when someone is reasonably well with regards to their mental health, can help prevent a mental health condition reaching crisis point. In my experience (and I realise that it's not the same for everyone), if practiced regularly, it can really help with stopping mental health difficulties getting right up there on the distress scale.
Not saying it's going to help everyone. Just my thoughts and experience.