I'm an ex-BlackBerry user and my 6 Plus is my first venture away from BlackBerry so, naturally, I'm a fan of BBM. I used to like it much more than I do now. I don't talk to most of 78 contacts and I talk to less of them than I did maybe a year ago when I switched platforms. I'm a part of a few groups and while I contemplated leaving BBM, I didn't want to lose my contacts or my groups. I still think BBM is best on a BlackBerry 10 device so for a long time I used it on my Z30. Now that it's back on my iPhone, I use it sometimes. One thing I noticed is that it's somewhat of a battery drain and always shows up on my Usage stats. Chatty groups are particularly problematic as there's no option to mute groups (something that's on the BlackBerry version). BBM would be awesome if I had just that one feature.
I don't want to part with BBM yet, but I won't be going back to BlackBerry any time soon.