I obtained the Ghosts of Saltmarsh book a few days ago. I haven't had a chance to delve into it in detail, but here are a few initial impressions.
First, it includes a solid 'mini-setting': Saltmarsh and the surrounding territory (the southernmost portion of the Kingdom of Keoland, roughly 18 leagues north-south, 25 leagues east-west). The setting has a lot of potential for expansion by DMs. Regions that can be fleshed out by creative DMs include: portions of the Hool Marshes and the Dreadwood, all of the Drowned Forest, the Silverstand Forest, some settlements (Seaton and Burle), and ... the ruins of the Tower of Zenopus! Yes, the sample dungeon from the original Holmes Basic D&D Set is given a location here, although it's not described in any detail. However, the brief description does align pretty closely with Holmes' description (but with 'Saltmarsh' replacing the original 'Portown').
The town of Saltmarsh is described in some detail. Three different factions -- each of which are represented in the ruling council -- are described ('traditionalists', 'loyalists', and the secret 'Scarlet Brotherhood' [!]). It looks like a great base for a low-level campaign: it has enough going on to keep the interest the players, including NPCs with whom to interact, but not too much to overwhelm them (or the DM).
The region is clearly situated within the World of Greyhawk, which is a refreshing change from the Forgotten Realms. (I'm not a FR-hater -- and I think that the Sword Coast region is a decent area for D&D campaigns -- but I find Greyhawk to be a much better setting overall. Also, it feels 'fresher' given how much has been produced already for the FR.) The Kingdom of Keoland and the Hold of the Sea Princes, including the conflict between them, are outlined. I hope that this signals that WotC will be expanding other parts of the WoG in future products.
Seven adventures are included. I'm only familiar with the original U1-3 AD&D modules (The Sinister Secret of Saltmarsh, Danger at Dunwater, and The Final Enemy).The others are from Dungeon magazine. All have sea/coast themes. Wisely, in my view, U3 is presented as a higher-level adventure (it states that it's for 4-6 seventh-level PCs), with two of the Dungeon adventures inserted between it and U2. The first adventure (U1) is for first-level characters, and the last one is for eleventh-level characters. I'm sceptical that there are enough adventures included in the book to use them exclusively for a 1-11 level campaign -- likely the DM will need to add a few additional ones in order to fill in some of the 'gaps'. But the book includes suggestions for incorporating some of the adventures from Tales from the Yawning Portal -- which is my other favourite WotC adventure book. Indeed, Saltmarsh and Portal are the kinds of adventure books that I (very much) prefer over WotC's usual 'adventure path' books. I find collections of adventures that can be run on their own or combined as the DM sees fit to be more useful than a series of tightly connected adventures meant to cover 10+ PC levels.
Aesthetically, the art and layout are really nice overall. The cover (by Grzegorz Rutkowski) is striking and dynamic. The maps (by Dyson Logos and Mike Schley) are attractive and (more importantly) clear.
So my initial impression of Ghosts of Saltmarsh is positive -- very positive. I'd love to see more products like this one and Yawning Portal from WotC for 5e D&D...