Just like any social platform, I think the advantage of using Meetup is that you'll get to a different audience than Facebook. In my case I've found the audience for Meetup to be smaller (i.e. far fewer RSVPs) but much more engaged, perhaps as a byproduct of the sheer volume of things on Facebook making it harder for yours to stand out. The breakdown of where attendees come from is roughly 30/50/20 Meetup/FB/Email, and the incremental time for me to post in all these places is very small.
The pricing is, as you say, odd, and I too have run into the frustration of expanding a single group to a neighboring city without wanting to drastically increase costs.
I think interest has been declining a bit, more "flakes" (sign up don't show) . The photo meets-ups I used to go to have shuttered, some of the programming ones are meeting more infrequently. The Social and board game meets-ups seem to be going ok. My partner is an organizer of a social group just pays the yearly fee to keep the group free.
Though in the past 10 years honestly participation ebs and flows. One organizer of "nerd fun boston" ran an event a couple events a week for a couple of years. He got a good group of regulars going to science lectures and history talks/walks around boston (he met his wife though the group). That group still hangs out occasionally, though more "virtually" through a Facebook group now, meet up doesn't do the social network as well I guess...
The site update recently looks nice, but seems not to have made it more functional.
I encourage people to go to meets-ups. Its nice to meet new people and get out of your social bubble. We meet mostly really nice people people going to these events.