Messantia is a good product. It's not such a good buy though, since the map is unispiring, the floorplans dull (and for me, superflous) and the adventure rather generic. Mainly, the box is full of air, which I think is bad form and really rather like fooling your buyers. So basically it's a lot of air considering how much you're payíng.
This having been said, if you can spare the cash, the main book is good material. While not as Conasque as Shadizar, I don't think Messantia CAN be as Conanesque. As it stands, it really reflects how I envision the city, and has some really neat gaming concepts built into it, like social combat and the merchant houses.
I'm glad I've got Messantia since it helps me a lot in my pirate campaign, and since it's a good read. But if my gaming club hadn't paid it for me, I'm not sure I'd bought it.