Disclaimer I haven't run all of Ruins of Mezro yet, just one of the one-shots.
So, Ruins of Mezro is awesome.
Well first off look at that cover layout, way more engaging then just art with title text. (not to knock other supplements, but come on)
Next you're going to read the dang thing and notice how well fleshed out and creative the ruined city is. It's got ancient curses, mysterious locations, piles of monsters, and a facinating backstory. This city feels like a living setting (which is funny on acount of how many undead are in there). There are so many ideas for other adventures in the city I get from reading about its crystal tower or its maze quarter. Even just the random encounters are fertile with cool story ideas, like that Death Knight guy or the wanderer.
Then we've got these NPCs, who are really interesting. Cimber shows up sure, but there's also a cursed Goliath warrior and an insecure cult leader who I love to run. These characters really pop because of their flaws and motivations.
Oh and there are lots of hooks from this supplement to Tomb of Annhilation and vise versa. That's what I was expecting, and granted I haven't read ToA so I can't say how well it will fit in practice but in theory it looks like it should be seemless.
Next we get the three adventures, which manage it pack some really intruiging role playing moments into their single-session format. That's something I think is hardest to pull off and it's doable in large part because of the well made NPCs. Frankly I think the adventures (which consist of around 10 of the packet's 40 pages) are the least interesting thing in here. Not because they're bad, they're not, they're great. The city they're set in however...well it's just way more interesting.
Finally we get some special monsters that show up and a cleric domain. This is fun looking stuff too but I haven't been able to test much of it.
In my opinion this kind of supplement is the best thing a DM can get ahold of. Not because it gives us epic plots or complex mechanics, but because it gives us a well thought-out framework with which to create. And isn't that the most fun part?