I am in an MBT group therapy Day Programme (which is mainly groupwork) and I've been doing it for 18 months. It's basically about developing a greater understanding of what is going on in your mind (feelings, emotions, thoughts etc.) and in the minds of others. It's good for people with BPD as (apparantly) we are not always able to do this well, especially in heightened emotional states. the therapy has not "cured" me (I'm still a work in progress) but I find it easier to recognise my state of mind and identify why I am feeling the way I am. It can be quite challenging in that it people in the groups are encouraged to say when they feel affected by what someone else has said in the group, but that has helped me to realise the effect I have on others and I can use what I have learnt in relationships outside of the therapy.
My self harm has reduced and I am able to tolerate difficult feelings more than I used to. The therapists encourage us to "stay with our feelings" which was a difficult thing to do at first, as usually, I would go straight to self harm, but gradually I found it easier.
I really hope it works out for you. Good luck!