Wow! A lot of negative reviews on here... I notice most of the naysayers tend to focus on an incident or anecdotal happening
My experiences have been pretty good especially if you link up with one that is active and has a few organizers and not just one or two. I n my homecity in Canada there is a wide selection to choose from and the larger general activities Meet-Ups have a wide following and a number of events. I've some good acquaintances and widened my social circle far more easily than striking up conversations in a bar.
I've traveled to other cities for and looked up local meet up group(s) and just show up and they've all been welcoming. Some may charge a couple dollars but I understand that there are costs associated with hosting the site and I'm fine with that.